Method and apparatus for determining oversensing in a medical device

ABSTRACT

A method and apparatus for determining oversensing of cardiac signals that includes a housing containing electronic circuitry, an electrode coupled to the electronic circuitry to sense cardiac signals, and a processor, positioned within the housing, to determine an oversensing characteristic associated with the cardiac signals sensed over a predetermined sensing window, and to identify oversensing in response to the determined oversensing characteristic.

CROSS-REFERENCE TO RELATED APPLICATION

Cross-reference is hereby made to the following commonly-assignedrelated U.S. Applications, which are incorporated herein by reference intheir entireties: U.S. application Ser. No. 11/742,618, filed May 1,2007, which published as U.S. Patent Application Publication No.2008/0275519, and is entitled “METHOD AND APPARATUS FOR DETECTINGARRHYTHMIAS IN A MEDICAL DEVICE”, by Ghanem et al.; U.S. applicationSer. No. 11/742,625, filed May 5, 2007, which published as U.S. PatentApplication Publication No. 2008/0275516, and is entitled “METHOD ANDAPPARATUS FOR DETECTING ARRHYTHMIAS IN A MEDICAL DEVICE”, by Ghanem etal.; and U.S. application Ser. No. 11/742,628, filed May 1, 2007, whichpublished as U.S. Patent Application Publication No. 2008/0275517, andis entitled “METHOD AND APPARATUS FOR DETECTING ARRHYTHMIAS IN A MEDICALDEVICE”, by Ghanem et al.

FIELD OF THE INVENTION

The present invention generally relates to an implantable medical devicesystem, and more particularly to a method and apparatus for detectingarrhythmias in a subcutaneous medical device.

BACKGROUND OF THE INVENTION

Many types of implantable medical devices (IMDs) have been implantedthat deliver relatively high-energy cardioversion and/or defibrillationshocks to a patient's heart when a malignant tachyarrhythmia, e.g.,ventricular tachycardia or ventricular fibrillation, is detected.Cardioversion shocks are typically delivered in synchrony with adetected R-wave when fibrillation detection criteria are met, whereasdefibrillation shocks are typically delivered when fibrillation criteriaare met and an R-wave cannot be discerned from the electrogram (EGM).

The current state of the art of ICDs or implantablepacemaker/cardioverter/defibrillators (PCDs) includes a full featuredset of extensive programmable parameters which includes multiplearrhythmia detection criteria, multiple therapy prescriptions (forexample, stimulation for pacing in the atrial, ventricular and/or bothchambers, bi-atrial and/or bi-ventricular pacing, arrhythmia overdriveor entrainment stimulation, and high level stimulation for cardioversionand/or defibrillation), extensive diagnostic capabilities and high speedtelemetry systems.

Current technology for the implantation of an IMD uses a transvenousapproach for cardiac electrodes and lead wires. The defibrillatorcanister/housing is generally implanted as an active can fordefibrillation and electrodes positioned in the heart are used forpacing, sensing and detection of arrhythmias.

Attempts are being made to identify patients who are asymptomatic byconventional measures but are nevertheless at risk of a future suddendeath episode. Current studies of patient populations, e.g., the MADITII and SCDHeFT studies, are establishing that there are large numbers ofpatients in any given population that are susceptible to sudden cardiacdeath, that they can be identified with some degree of certainty andthat they are candidates for a prophylactic implantation of adefibrillator (often called primary prevention).

One option proposed for this patient population is to implant aprophylactic subcutaneous implantable device (SubQ device). As SubQdevice technology evolves, it may develop a clear and distinct advantageover non-SubQ devices. For example, the SubQ device does not requireleads to be placed in the bloodstream. Accordingly, complicationsarising from leads placed in the cardiovasculature environment areeliminated. Further, endocardial lead placement is not possible withpatients who have a mechanical heart valve implant and is not generallyrecommended for pediatric cardiac patients. For these and other reasons,a SubQ device may be preferred over an ICD.

There are technical challenges associated with the operation of a SubQdevice. For example, SubQ device sensing is challenged by the presenceof muscle artifact, respiration and other physiological signal sources.This is particularly because the SubQ device is limited to far-fieldsensing since there are no intracardial or epicardial electrodes in asubcutaneous system. Further, sensing of atrial activation fromsubcutaneous electrodes is limited since the atria represent a smallmuscle mass and the atrial signals are not sufficiently detectabletransthoracically.

Yet another challenge could occur in situations where it is desirable tocombine a SubQ device with an existing pacemaker (IPG) in a patient.While this may be desirable in a case where an IPG patient may need adefibrillator, a combination implant of a SubQ device and an IPG mayresult in inappropriate therapy by the SubQ device, which may pace orshock based on spikes from the IPG. Specifically, each time the IPGemits a pacing stimulus, the SubQ device may interpret it as a genuinecardiac beat. The result can be over-counting beats from the atrium,ventricles or both; or, because of the larger pacing spikes, sensing ofarrhythmic signals (which are typically much smaller in amplitude) maybe compromised.

Therefore, for these and other reasons, a need exists for an improvedmethod and apparatus to reliably sense and detect arrhythmias in asubcutaneous device, while rejecting noise and other physiologicsignals.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects and features of the present invention will be appreciated as thesame becomes better understood by reference to the following detaileddescription of the embodiments of the invention when considered inconnection with the accompanying drawings, wherein:

FIGS. 1 and 2 are schematic diagrams of an exemplary subcutaneous devicein which the present invention may be usefully practiced;

FIG. 3 is an exemplary schematic diagram of electronic circuitry withina hermetically sealed housing of a subcutaneous device of the presentinvention;

FIG. 4 is a schematic diagram of signal processing aspects of asubcutaneous device according to an exemplary embodiment of the presentinvention;

FIG. 5 is a state diagram of detection of arrhythmias in a subcutaneousdevice according to an embodiment of the present invention;

FIG. 6 is a flow chart of a method for detecting arrhythmias in asubcutaneous device according to an embodiment of the present invention;

FIGS. 7A-7J are flow charts of a method for detecting arrhythmias in asubcutaneous device according to an embodiment of the present invention;

FIG. 8 is a graphical representation of sensing of cardiac activityaccording to an embodiment of the present invention;

FIG. 9A is a graphical representation of a determination of whether asignal is corrupted by muscle noise according to an embodiment of thepresent invention;

FIG. 9B is a flowchart of a method of determining whether a signal iscorrupted by muscle noise according to an embodiment of the presentinvention;

FIG. 9C is a flowchart of a method of determining whether a signal iscorrupted by muscle noise according to an embodiment of the presentinvention;

FIG. 10 is a graphical representation of a VF shock zone according to anembodiment of the present invention;

FIGS. 11A and 11B are graphical representations of the determination ofwhether an event is within a shock zone according to an embodiment ofthe present invention;

FIG. 12 is a graphical representation of a shock zone according to anembodiment of the present invention;

FIG. 13 is a graphical representation of the determination of whether anevent is within a shock zone according to an embodiment of the presentinvention;

FIGS. 14A-14C are graphical representations illustrating the occurrenceof oversensing due to a slow monomorphic ventricular tachycardia with awide QRS complex;

FIG. 15 is a flowchart of a method for detecting cardiac events in amedical device according to an embodiment of the present invention;

FIGS. 16A and 16B are flowcharts of a method of determining whetheroversensing has occurred according to an embodiment of the presentinvention;

FIG. 17 is a flowchart of a method of determining whether oversensinghas occurred according to an embodiment of the present invention;

FIG. 18 is a flowchart of a method of determining whether oversensinghas occurred according to an embodiment of the present invention;

FIGS. 19A and 19B are graphical representations of determining acorrected heart rate in response to oversensing according to anembodiment of the present invention;

FIGS. 20A and 20B are flowcharts of a method of determining a correctedheart rate in response to oversensing according to an embodiment of thepresent invention;

FIG. 21 is a flowchart of a method for determining a corrected rateaccording to an embodiment of the present invention;

FIG. 22 is a flowchart of a method of determining a corrected heart ratein response to oversensing according to an embodiment of the presentinvention;

FIG. 23 is an exemplary schematic diagram of a buffer of RR intervalsgenerated according to an embodiment of the present invention;

FIG. 24 is a flowchart of a method for determining a corrected rateaccording to an embodiment of the present invention;

FIG. 25 is an exemplary schematic diagram of a buffer of RR intervalsgenerated according to an embodiment of the present invention; and

FIG. 26 is a flowchart of a method for detecting cardiac events in amedical device according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram of an exemplary subcutaneous device inwhich the present invention may be usefully practiced. As illustrated inFIG. 1, a subcutaneous device 14 according to an embodiment of thepresent invention is subcutaneously implanted outside the ribcage of apatient 12, anterior to the cardiac notch. Further, a subcutaneoussensing and cardioversion/defibrillation therapy delivery lead 18 inelectrical communication with subcutaneous device 14 is tunneledsubcutaneously into a location adjacent to a portion of a latissimusdorsi muscle of patient 12. Specifically, lead 18 is tunneledsubcutaneously from the median implant pocket of the subcutaneous device14 laterally and posterially to the patient's back to a locationopposite the heart such that the heart 16 is disposed between thesubcutaneous device 14 and the distal electrode coil 24 and distalsensing electrode 26 of lead 18.

It is understood that while the subcutaneous device 14 is shownpositioned through loose connective tissue between the skin and musclelayer of the patient, the term “subcutaneous device” is intended toinclude a device that can be positioned in the patient to be implantedusing any non-intravenous location of the patient, such as below themuscle layer or within the thoracic cavity, for example.

Further referring to FIG. 1, a programmer 20 is shown in telemetriccommunication with subcutaneous device 14 by an RF communication link22. Communication link 22 may be any appropriate RF link such asBluetooth, WiFi, MICS, or as described in U.S. Pat. No. 5,683,432“Adaptive Performance-Optimizing Communication System for Communicatingwith an Implantable Medical Device” to Goedeke, et al and incorporatedherein by reference in its entirety.

Subcutaneous device 14 includes a housing 15 that may be constructed ofstainless steel, titanium or ceramic as described in U.S. Pat. No.4,180,078 “Lead Connector for a Body Implantable Stimulator” to Andersonand U.S. Pat. No. 5,470,345 “Implantable Medical Device withMulti-layered Ceramic Enclosure” to Hassler, et al, both incorporatedherein by reference in their entireties. The electronics circuitry ofSubQ ICD 14 may be incorporated on a polyimide flex circuit, printedcircuit board (PCB) or ceramic substrate with integrated circuitspackaged in leadless chip carriers and/or chip scale packaging (CSP).

Subcutaneous lead 18 includes a distal defibrillation coil electrode 24,a distal sensing electrode 26, an insulated flexible lead body and aproximal connector pin 27 (shown in FIG. 2) for connection to thehousing 15 of the subcutaneous device 14 via a connector 25. Inaddition, one or more electrodes 28 (shown in FIG. 2) are positionedalong the outer surface of the housing to form a housing-basedsubcutaneous electrode array (SEA). Distal sensing electrode 26 is sizedappropriately to match the sensing impedance of the housing-basedsubcutaneous electrode array.

It is understood that while device 14 is shown with electrodes 28positioned on housing 15, according to an embodiment of the presentinvention electrodes 28 may be alternatively positioned along one ormore separate leads connected to device 14 via connector 25.

Continuing with FIG. 2, electrodes 28 are welded into place on theflattened periphery of the housing 15. In the embodiment depicted inthis figure, the complete periphery of the SubQ ICD may be manufacturedto have a slightly flattened perspective with rounded edges toaccommodate the placement of the electrodes 28. The electrodes 28 arewelded to housing 15 (to preserve hermaticity) and are connected viawires (not shown) to electronic circuitry (described herein below)inside housing 15. Electrodes 28 may be constructed of flat plates, oralternatively, may be spiral electrodes as described in U.S. Pat. No.6,512,940 “Subcutaneous Spiral Electrode for Sensing Electrical Signalsof the Heart” to Brabec, et al and mounted in a non-conductive surroundshroud as described in U.S. Pat. No. 6,522,915 “Surround ShroudConnector and Electrode Housings for a Subcutaneous Electrode Array andLeadless ECGs” to Ceballos, et al and U.S. Pat. No. 6,622,046“Subcutaneous Sensing Feedthrough/Electrode Assembly” to Fraley, et al,all incorporated herein by reference in their entireties. The electrodes28 of FIG. 2 can be positioned to form orthogonal or equilateral signalvectors, for example.

The electronic circuitry employed in subcutaneous device 14 can take anyof the known forms that detect a tachyarrhythmia from the sensed ECG andprovide cardioversion/defibrillation shocks as well as post-shock pacingas needed while the heart recovers. A simplified block diagram of suchcircuitry adapted to function employing the first and secondcardioversion-defibrillation electrodes as well as the ECG sensing andpacing electrodes described herein below is set forth in FIG. 3. It willbe understood that the simplified block diagram does not show all of theconventional components and circuitry of such devices including digitalclocks and clock lines, low voltage power supply and supply lines forpowering the circuits and providing pacing pulses or telemetry circuitsfor telemetry transmissions between the device 14 and externalprogrammer 20.

FIG. 3 is an exemplary schematic diagram of electronic circuitry withina hermetically sealed housing of a subcutaneous device according to anembodiment of the present invention. As illustrated in FIG. 3,subcutaneous device 14 includes a low voltage battery 153 coupled to apower supply (not shown) that supplies power to the circuitry of thesubcutaneous device 14 and the pacing output capacitors to supply pacingenergy in a manner well known in the art. The low voltage battery 153may be formed of one or two conventional LiCF_(x), LiMnO₂ or LiI₂ cells,for example. The subcutaneous device 14 also includes a high voltagebattery 112 that may be formed of one or two conventional LiSVO orLiMnO₂ cells. Although two both low voltage battery and a high voltagebattery are shown in FIG. 3, according to an embodiment of the presentinvention, the device 14 could utilize a single battery for both highand low voltage uses.

Further referring to FIG. 3, subcutaneous device 14 functions arecontrolled by means of software, firmware and hardware thatcooperatively monitor the ECG, determine when acardioversion-defibrillation shock or pacing is necessary, and deliverprescribed cardioversion-defibrillation and pacing therapies. Thesubcutaneous device 14 may incorporate circuitry set forth in commonlyassigned U.S. Pat. Nos. 5,163,427 “Apparatus for Delivering Single andMultiple Cardioversion and Defibrillation Pulses” to Keimel and5,188,105 “Apparatus and Method for Treating a Tachyarrhythmia” toKeimel for selectively delivering single phase, simultaneous biphasicand sequential biphasic cardioversion-defibrillation shocks typicallyemploying ICD IPG housing electrodes 28 coupled to the COMMON output 123of high voltage output circuit 140 and cardioversion-defibrillationelectrode 24 disposed posterially and subcutaneously and coupled to theHVI output 113 of the high voltage output circuit 140. Outputs 132 ofFIG. 3 is coupled to sense electrode 26.

The cardioversion-defibrillation shock energy and capacitor chargevoltages can be intermediate to those supplied by ICDs having at leastone cardioversion-defibrillation electrode in contact with the heart andmost AEDs having cardioversion-defibrillation electrodes in contact withthe skin. The typical maximum voltage necessary for ICDs using mostbiphasic waveforms is approximately 750 Volts with an associated maximumenergy of approximately 40 Joules. The typical maximum voltage necessaryfor AEDs is approximately 2000-5000 Volts with an associated maximumenergy of approximately 200-360 Joules depending upon the model andwaveform used. The subcutaneous device 14 of the present invention usesmaximum voltages in the range of about 300 to approximately 1000 Voltsand is associated with energies of approximately 25 to 150 joules ormore. The total high voltage capacitance could range from about 50 toabout 300 microfarads. Such cardioversion-defibrillation shocks are onlydelivered when a malignant tachyarrhythmia, e.g., ventricularfibrillation is detected through processing of the far field cardiac ECGemploying the detection algorithms as described herein below.

In FIG. 3, sense amp 190 in conjunction with pacer/device timing circuit178 processes the far field ECG sense signal that is developed across aparticular ECG sense vector defined by a selected pair of thesubcutaneous electrodes 24, 26 and 28, or, optionally, a virtual signal(i.e., a mathematical combination of two vectors) if selected. Theselection of the sensing electrode pair is made through the switchmatrix/MUX 191 in a manner to provide the most reliable sensing of theECG signal of interest, which would be the R wave for patients who arebelieved to be at risk of ventricular fibrillation leading to suddendeath. The far field ECG signals are passed through the switchmatrix/MUX 191 to the input of the sense amplifier 190 that, inconjunction with pacer/device timing circuit 178, evaluates the sensedEGM. Bradycardia, or asystole, is typically determined by an escapeinterval timer within the pacer timing circuit 178 and/or the controlcircuit 144. Pace Trigger signals are applied to the pacing pulsegenerator 192 generating pacing stimulation when the interval betweensuccessive R-waves exceeds the escape interval. Bradycardia pacing isoften temporarily provided to maintain cardiac output after delivery ofa cardioversion-defibrillation shock that may cause the heart to slowlybeat as it recovers back to normal function. Sensing subcutaneous farfield signals in the presence of noise may be aided by the use ofappropriate denial and extensible accommodation periods as described inU.S. Pat. No. 6,236,882 “Noise Rejection for Monitoring ECGs” to Lee, etal and incorporated herein by reference in its' entirety.

Detection of a malignant tachyarrhythmia is determined in the controlcircuit 144 as a function of the intervals between R-wave sense eventsignals that are output from the pacer/device timing 178 and senseamplifier circuit 190 to the timing and control circuit 144. It shouldbe noted that the present invention utilizes not only interval basedsignal analysis method but also supplemental sensors and morphologyprocessing method and apparatus as described herein below.

Supplemental sensors such as tissue color, tissue oxygenation,respiration, patient activity and the like may be used to contribute tothe decision to apply or withhold a defibrillation therapy as describedgenerally in U.S. Pat. No. 5,464,434 “Medical Interventional DeviceResponsive to Sudden Hemodynamic Change” to Alt and incorporated hereinby reference in its entirety. Sensor processing block 194 providessensor data to microprocessor 142 via data bus 146. Specifically,patient activity and/or posture may be determined by the apparatus andmethod as described in U.S. Pat. No. 5,593,431 “Medical ServiceEmploying Multiple DC Accelerometers for Patient Activity and PostureSensing and Method” to Sheldon and incorporated herein by reference inits entirety. Patient respiration may be determined by the apparatus andmethod as described in U.S. Pat. No. 4,567,892 “Implantable CardiacPacemaker” to Plicchi, et al and incorporated herein by reference in itsentirety. Patient tissue oxygenation or tissue color may be determinedby the sensor apparatus and method as described in U.S. Pat. No.5,176,137 to Erickson, et al and incorporated herein by reference in itsentirety. The oxygen sensor of the '137 patent may be located in thesubcutaneous device pocket or, alternatively, located on the lead 18 toenable the sensing of contacting or near-contacting tissue oxygenationor color.

Certain steps in the performance of the detection algorithm criteria arecooperatively performed in microcomputer 142, including microprocessor,RAM and ROM, associated circuitry, and stored detection criteria thatmay be programmed into RAM via a telemetry interface (not shown)conventional in the art. Data and commands are exchanged betweenmicrocomputer 142 and timing and control circuit 144, pacertiming/amplifier circuit 178, and high voltage output circuit 140 via abidirectional data/control bus 146. The pacer timing/amplifier circuit178 and the control circuit 144 are clocked at a slow clock rate. Themicrocomputer 142 is normally asleep, but is awakened and operated by afast clock by interrupts developed by each R-wave sense event, onreceipt of a downlink telemetry programming instruction or upon deliveryof cardiac pacing pulses to perform any necessary mathematicalcalculations, to perform tachycardia and fibrillation detectionprocedures, and to update the time intervals monitored and controlled bythe timers in pacer/device timing circuitry 178.

When a malignant tachycardia is detected, high voltage capacitors 156,158, 160, and 162 are charged to a pre-programmed voltage level by ahigh-voltage charging circuit 164. It is generally consideredinefficient to maintain a constant charge on the high voltage outputcapacitors 156, 158, 160, 162. Instead, charging is initiated whencontrol circuit 144 issues a high voltage charge command HVCHG deliveredon line 145 to high voltage charge circuit 164 and charging iscontrolled by means of bidirectional control/data bus 166 and a feedbacksignal VCAP from the HV output circuit 140. High voltage outputcapacitors 156, 158, 160 and 162 may be of film, aluminum electrolyticor wet tantalum construction.

The negative terminal of high voltage battery 112 is directly coupled tosystem ground. Switch circuit 114 is normally open so that the positiveterminal of high voltage battery 112 is disconnected from the positivepower input of the high voltage charge circuit 164. The high voltagecharge command HVCHG is also conducted via conductor 149 to the controlinput of switch circuit 114, and switch circuit 114 closes in responseto connect positive high voltage battery voltage EXT B+ to the positivepower input of high voltage charge circuit 164. Switch circuit 114 maybe, for example, a field effect transistor (FET) with itssource-to-drain path interrupting the EXT B+ conductor 118 and its gatereceiving the HVCHG signal on conductor 145. High voltage charge circuit164 is thereby rendered ready to begin charging the high voltage outputcapacitors 156, 158, 160, and 162 with charging current from highvoltage battery 112.

High voltage output capacitors 156, 158, 160, and 162 may be charged tovery high voltages, e.g., 300-1000V, to be discharged through the bodyand heart between the electrode pair of subcutaneouscardioversion-defibrillation electrodes 113 and 123. The details of thevoltage charging circuitry are also not deemed to be critical withregard to practicing the present invention; one high voltage chargingcircuit believed to be suitable for the purposes of the presentinvention is disclosed. High voltage capacitors 156, 158, 160 and 162may be charged, for example, by high voltage charge circuit 164 and ahigh frequency, high-voltage transformer 168 as described in detail incommonly assigned U.S. Pat. No. 4,548,209 “Energy Converter forImplantable Cardioverter” to Wielders, et al. Proper charging polaritiesare maintained by diodes 170, 172, 174 and 176 interconnecting theoutput windings of high-voltage transformer 168 and the capacitors 156,158, 160, and 162. As noted above, the state of capacitor charge ismonitored by circuitry within the high voltage output circuit 140 thatprovides a VCAP, feedback signal indicative of the voltage to the timingand control circuit 144. Timing and control circuit 144 terminates thehigh voltage charge command HVCHG when the VCAP signal matches theprogrammed capacitor output voltage, i.e., thecardioversion-defibrillation peak shock voltage.

Control circuit 144 then develops first and second control signalsNPULSE 1 and NPULSE 2, respectively, that are applied to the highvoltage output circuit 140 for triggering the delivery of cardiovertingor defibrillating shocks. In particular, the NPULSE 1 signal triggersdischarge of the first capacitor bank, comprising capacitors 156 and158. The NPULSE 2 signal triggers discharge of the first capacitor bankand a second capacitor bank, comprising capacitors 160 and 162. It ispossible to select between a plurality of output pulse regimes simply bymodifying the number and time order of assertion of the NPULSE 1 andNPULSE 2 signals. The NPULSE 1 signals and NPULSE 2 signals may beprovided sequentially, simultaneously or individually. In this way,control circuitry 144 serves to control operation of the high voltageoutput stage 140, which delivers high energycardioversion-defibrillation shocks between the pair of thecardioversion-defibrillation electrodes 113 and 123 coupled to the HV-1and COMMON output as shown in FIG. 3.

Thus, subcutaneous device 14 monitors the patient's cardiac status andinitiates the delivery of a cardioversion-defibrillation shock throughthe cardioversion-defibrillation electrodes 24 and 28 in response todetection of a tachyarrhythmia requiring cardioversion-defibrillation.The high HVCHG signal causes the high voltage battery 112 to beconnected through the switch circuit 114 with the high voltage chargecircuit 164 and the charging of output capacitors 156, 158, 160, and 162to commence. Charging continues until the programmed charge voltage isreflected by the VCAP signal, at which point control and timing circuit144 sets the HVCHG signal low terminating charging and opening switchcircuit 114. Typically, the charging cycle takes only fifteen to twentyseconds, and occurs very infrequently. The subcutaneous device 14 can beprogrammed to attempt to deliver cardioversion shocks to the heart inthe manners described above in timed synchrony with a detected R-wave orcan be programmed or fabricated to deliver defibrillation shocks to theheart in the manners described above without attempting to synchronizethe delivery to a detected R-wave. Episode data related to the detectionof the tachyarrhythmia and delivery of the cardioversion-defibrillationshock can be stored in RAM for uplink telemetry transmission to anexternal programmer as is well known in the art to facilitate indiagnosis of the patient's cardiac state. A patient receiving the device14 on a prophylactic basis would be instructed to report each suchepisode to the attending physician for further evaluation of thepatient's condition and assessment for the need for implantation of amore sophisticated ICD.

Subcutaneous device 14 desirably includes telemetry circuit (not shownin FIG. 3), so that it is capable of being programmed by means ofexternal programmer 20 via a 2-way telemetry link 22 (shown in FIG. 1).Uplink telemetry allows device status and diagnostic/event data to besent to external programmer 20 for review by the patient's physician.Downlink telemetry allows the external programmer via physician controlto allow the programming of device function and the optimization of thedetection and therapy for a specific patient. Programmers and telemetrysystems suitable for use in the practice of the present invention havebeen well known for many years. Known programmers typically communicatewith an implanted device via a bidirectional radio-frequency telemetrylink, so that the programmer can transmit control commands andoperational parameter values to be received by the implanted device, sothat the implanted device can communicate diagnostic and operationaldata to the programmer. Programmers believed to be suitable for thepurposes of practicing the present invention include the Models 9790 andCareLink® programmers, commercially available from Medtronic, Inc.,Minneapolis, Minn.

Various telemetry systems for providing the necessary communicationschannels between an external programming unit and an implanted devicehave been developed and are well known in the art. Telemetry systemsbelieved to be suitable for the purposes of practicing the presentinvention are disclosed, for example, in the following U.S. Pat. No.5,127,404 to Wyborny et al. entitled “Telemetry Format for ImplantedMedical Device”; U.S. Pat. No. 4,374,382 to Markowitz entitled “MarkerChannel Telemetry System for a Medical Device”; and U.S. Pat. No.4,556,063 to Thompson et al. entitled “Telemetry System for a MedicalDevice”. The Wyborny et al. '404, Markowitz '382, and Thompson et al.'063 patents are commonly assigned to the assignee of the presentinvention, and are each hereby incorporated by reference herein in theirrespective entireties.

FIG. 4 is a schematic diagram of signal processing aspects of asubcutaneous device according to an exemplary embodiment of the presentinvention. The transthoracic ECG signal (ECG1) detected between thedistal electrode 26 of subcutaneous lead 18 and one of electrodes 28positioned on the subcutaneous device 14 are amplified and bandpassfiltered (2.5-105 Hz) by pre-amplifiers 202 and 206 located in Sense Amp190 of FIG. 3. The amplified EGM signals are directed to A/D converters210 and 212, which operate to sample the time varying analog EGM signaland digitize the sampled points. The digital output of A/D converters210 and 212 are applied to temporary buffers/control logic, which shiftsthe digital data through its stages in a FIFO manner under the controlof Pacer/Device Timing block 178 of FIG. 3. Virtual Vector block 226selects one housing-based ECG signal (ECG2) from any pair of electrodes28 as described, for example, in U.S. Pat. No. 5,331,966 “SubcutaneousMulti-Electrode Sensing System, Method and Pacer” to Bennett, et al or,alternatively, generates a virtual vector signal under control ofMicroprocessor 142 and Control block 144 as described in U.S. Pat. No.6,505,067 “System and Method for Deriving Virtual ECG or EGM Signal” toLee, et al; both patents incorporated herein by reference in theirentireties. ECG1 and ECG2 vector selection may be selected by thepatient's physician and programmed via telemetry link 22 from programmer20.

According to an embodiment of the present invention, in order toautomatically select the preferred ECG vector set, it is necessary tohave an index of merit upon which to rate the quality of the signal.“Quality” is defined as the signal's ability to provide accurate heartrate estimation and accurate morphological waveform separation betweenthe patient's usual sinus rhythm and the patient's ventriculartachyarrhythmia.

Appropriate indices may include R-wave amplitude, R-wave peak amplitudeto waveform amplitude between R-waves (i.e., signal to noise ratio), lowslope content, relative high versus low frequency power, mean frequencyestimation, probability density function, or some combination of thesemetrics.

Automatic vector selection might be done at implantation or periodically(daily, weekly, monthly) or both. At implant, automatic vector selectionmay be initiated as part of an automatic device turn-on procedure thatperforms such activities as measure lead impedances and batteryvoltages. The device turn-on procedure may be initiated by theimplanting physician (e.g., by pressing a programmer button) or,alternatively, may be initiated automatically upon automatic detectionof device/lead implantation. The turn-on procedure may also use theautomatic vector selection criteria to determine if ECG vector qualityis adequate for the current patient and for the device and leadposition, prior to suturing the subcutaneous device 14 device in placeand closing the incision. Such an ECG quality indicator would allow theimplanting physician to maneuver the device to a new location ororientation to improve the quality of the ECG signals as required. Thepreferred ECG vector or vectors may also be selected at implant as partof the device turn-on procedure. The preferred vectors might be thosevectors with the indices that maximize rate estimation and detectionaccuracy. There may also be an a priori set of vectors that arepreferred by the physician, and as long as those vectors exceed someminimum threshold, or are only slightly worse than some other moredesirable vectors, the a priori preferred vectors are chosen. Certainvectors may be considered nearly identical such that they are not testedunless the a priori selected vector index falls below some predeterminedthreshold.

Depending upon metric power consumption and power requirements of thedevice, the ECG signal quality metric may be measured on the range ofvectors (or alternatively, a subset) as often as desired. Data may begathered, for example, on a minute, hourly, daily, weekly or monthlybasis. More frequent measurements (e.g., every minute) may be averagedover time and used to select vectors based upon susceptibility ofvectors to occasional noise, motion noise, or EMI, for example.

Alternatively, the subcutaneous device 14 may have an indicator/sensorof patient activity (piezo-resistive, accelerometer, impedance, or thelike) and delay automatic vector measurement during periods of moderateor high patient activity to periods of minimal to no activity. Onerepresentative scenario may include testing/evaluating ECG vectors oncedaily or weekly while the patient has been determined to be asleep(using an internal clock (e.g., 2:00 am) or, alternatively, infer sleepby determining the patient's position (via a 2- or 3-axis accelerometer)and a lack of activity).

If infrequent automatic, periodic measurements are made, it may also bedesirable to measure noise (e.g., muscle, motion, EMI, etc.) in thesignal and postpone the vector selection measurement when the noise hassubsided.

Subcutaneous device 14 may optionally have an indicator of the patient'sposture (via a 2- or 3-axis accelerometer). This sensor may be used toensure that the differences in ECG quality are not simply a result ofchanging posture/position. The sensor may be used to gather data in anumber of postures so that ECG quality may be averaged over thesepostures or, alternatively, selected for a preferred posture.

In the preferred embodiment, vector quality metric calculations wouldoccur a number of times over approximately 1 minute, once per day, foreach vector. These values would be averaged for each vector over thecourse of one week. Averaging may consist of a moving average orrecursive average depending on time weighting and memory considerations.In this example, the preferred vector(s) would be selected once perweek.

Continuing with FIG. 4, a diagnostic channel 228 receives a programmableselected ECG signal from the housing based subcutaneous electrodes andthe transthoracic ECG from the distal electrode 26 on lead 18. Block 238compresses the digital data, the data is applied to temporarybuffers/control logic 218 which shifts the digital data through itsstages in a FIFO manner under the control of Pacer/Device Timing block178 of FIG. 3, and the data is then stored in SRAM block 244 via directmemory access block 242.

The two selected ECG signals (ECG1 and ECG2) are additionally used toprovide R-wave interval sensing via ECG sensing block 230. IIR notchfilter block 246 provides 50/60 Hz notch filtering. A rectifier andauto-threshold block 248 provides R-wave event detection as described inU.S. Pat. No. 5,117,824 “Apparatus for Monitoring Electrical PhysiologicSignals” to Keimel, et al; publication WO2004023995 “Method andApparatus for Cardiac R-wave Sensing in a Subcutaneous ECG Waveform” toCao, et al and U.S. Publication No. 2004/0260350 “Automatic EGMAmplitude Measurements During Tachyarrhythmia Episodes” to Brandstetter,et al, all incorporated herein by reference in their entireties. Therectifier of block 248 performs full wave rectification on theamplified, narrowband signal from bandpass filter 246. A programmablefixed threshold (percentage of peak value), a moving average or, morepreferably, an auto-adjusting threshold is generated as described in the'824 patent or '350 publication. In these references, following adetected depolarization, the amplifier is automatically adjusted so thatthe effective sensing threshold is set to be equal to a predeterminedportion of the amplitude of the sensed depolarization, and the effectivesensing threshold decays thereafter to a lower or base-sensingthreshold. A comparator in block 248 determines signal crossings fromthe rectified waveform and auto-adjusting threshold signal. A timerblock 250 provides R-wave to R-wave interval timing for subsequentarrhythmia detection (to be described herein below). The heart rateestimation is derived from the last 12 R-R intervals (e.g., by a mean,trimmed mean, or median, for example), with the oldest data value beingremoved as a new data value is added.

FIG. 5 is a schematic diagram of a rectifier and auto-threshold unit ina subcutaneous device according to an embodiment of the presentinvention. Waveform 402 depicts a typical subcutaneous ECG waveform andwaveform 404 depicts the same waveform after filtering andrectification. A time dependant threshold 406 allows a more sensitivesensing threshold temporally with respect to the previous sensed R-wave.Sensed events 408 indicate when the rectified ECG signal 404 exceeds theauto-adjusting threshold and a sensed event has occurred.

Returning to FIG. 4, the subcutaneous ECG signal (ECG1) is applied toECG morphology block 232, filtered by a 2-pole 23 Hz low pass filter 252and evaluated by DSP microcontroller 254 under control of programinstructions stored in System Instruction RAM 258. ECG morphology isused for subsequent rhythm detection/determination (to be describedherein below).

FIG. 6 is a state diagram of detection of arrhythmias in a medicaldevice according to an embodiment of the present invention. Asillustrated in FIG. 6, during normal operation, the device 14 is in anot concerned state 302, described in more detail herein below, duringwhich R-wave intervals are being evaluated to identify periods of rapidrates and/or the presence of asystole. Upon detection of short R-waveintervals simultaneously in both ECG leads, indicative of an event that,if confirmed, may require the delivery of therapy, the device 14transitions from the not concerned state 302 to a concerned state 304,described in more detail herein below. In the concerned state 304 thedevice 14 evaluates a predetermined window of ECG signals to determinethe likelihood that the signal is corrupted with noise and todiscriminate rhythms requiring shock therapy from those that do notrequire shock therapy, using a combination of R-wave intervals and ECGsignal morphology information.

If a rhythm requiring shock therapy continues to be detected while inthe concerned state 304, the device 14 transitions from the concernedstate 304 to an armed state 306, described in more detail herein below.If a rhythm requiring shock therapy is no longer detected while thedevice is in the concerned state 304 and the R-wave intervals aredetermined to no longer be short, the device 14 returns to the notconcerned state 302. However, if a rhythm requiring shock therapy is nolonger detected while the device is in the concerned state 304, but theR-wave intervals continue to be detected as being short, processingcontinues in the concerned state 304.

In the armed state 306, the device 14 charges the high voltage shockingcapacitors and continues to monitor R-wave intervals and ECG signalmorphology for spontaneous termination. If spontaneous termination ofthe rhythm requiring shock therapy occurs, the device 14 returns to thenot concerned state 302. If the rhythm requiring shock therapy is stilldetermined to be occurring once the charging of the capacitors iscompleted, the device 14 transitions from the armed state 306 to a shockstate 308, described in more detail herein below.

In the shock state 308, the device 14 delivers a shock and returns tothe armed state 306 to evaluate the success of the therapy delivered.

FIGS. 7A-7I are flow charts of a method for detecting arrhythmias in asubcutaneous device according to an embodiment of the present invention.As illustrated in FIG. 7A, device 14 continuously evaluates the twochannels ECG1 and ECG2 associated with two predetermined electrodevectors to when sensed events occur. For example, the electrode vectorsfor the two channels ECG1 and ECG2 may include a horizontal vectorselected between two of the electrodes 28 (ECG2) located along thehousing of the device 14 as one electrode vector, while the otherelectrode vector is a front to back vector selected between the distalelectrode 26 (ECG1) and one of the subcutaneous electrodes 28, forexample. The input signal from each channel ECG1 and ECG2 ispre-processed and rectified, and an adaptive auto-adjusting threshold isthen applied. According to an embodiment of the present invention, asensed event is determined to have occurred, for example, whenever therising edge of the filtered ECG crosses the threshold.

FIG. 8 is a graphical representation of sensing of cardiac activityaccording to an embodiment of the present invention. In particular, thepresent invention utilizes an adaptive auto-adjusting threshold 401during the R-wave sensing of Block 322 that includes a first thresholdlevel 403, a second threshold level 405, a third threshold level 407 anda fourth threshold level 409. An example of an auto-adjusting thresholdis described, for example, in commonly assigned U.S. Patent ApplicationPublication No. 2004/0049120, to Cao et al., filed Sep. 11, 2002,incorporated herein by reference in its entirety. Once there is a sensedevent, which occurs whenever the rising edge of the rectified filteredECG 411 crosses the threshold level, in this case threshold 403,indicated by marker 410, the threshold 401 is adjusted to the secondthreshold level 405, which is a first predetermined percentage of a peakamplitude 412 of the rectified filtered ECG 411, such as 65 percent, forexample.

A blanking period 414 (nominally 150 ms, for example) prevents doublecounting of R-waves. During blanking period 414, the threshold 401continues to track the predetermined percentage of rectified filteredECG 411 until peak 412 is detected. Threshold 401 is held at the secondthreshold level 405 during a threshold hold time period 416 (nominally100 ms, for example) starting from the peak 412 location to preventT-wave oversensing by delaying the linear decay. Threshold 401 thendecays at a first predetermined rate, such as 35% of peak 412 persecond, for example, until threshold 401 reaches the third thresholdlevel 407, which is a second predetermined percentage of peak amplitude412 (nominally 30%, for example). Threshold 401 is held at the thirdthreshold level 407 until a step drop time 418 from the sensed event 410(1.5 sec, for example) has expired. Once the step drop time 418 hasexpired, the threshold 401 is instantaneously set at the fourththreshold level 409 and begins to decay at a second predetermined rate,such as 20% of peak 412 per second, for example. The threshold 401continues to decay linearly at the second predetermined rate until thethreshold 401 reaches the first threshold level 403. At no time can thethreshold 401 become less than the first threshold level 403.

The step drop time 418 allows abrupt adjustment of the threshold 401 inorder to accommodate sensing of sudden reductions in R-wave amplitudes.The second predetermined rate associated with the linear decay is set ata rate that prevents oversensing of P-waves while maintaining adequatedecay for sensing sudden drops in R-waves. If, at any time throughoutthis threshold adjustment process, a sensed event re-occurs outsideblanking period 414, then the threshold 401 is adjusted to the secondthreshold level 405, and the threshold adjustment process is repeated.

According to an embodiment of the present invention, the nominalsettings for the R-wave detector parameters may be set, for example,with the first threshold level being 25 microvolts, the second thresholdlevel, third threshold level and fourth threshold level being set as 65,30 and 20 percent of the peak amplitude 412, respectively, blankingperiod 414 being set as 150 milliseconds, threshold hold time 416 beingset as 100 milliseconds, and a maximum threshold level being 650microvolts. These nominal settings may differ between the anteriorhousing-based bipolar ECG and the front to back ECG in order to accountfor the expected difference in amplitude and noise characteristics forthose vectors.

The R-wave sensing described above is applied to each ECG channel ECG1and ECG2 independently. According to the present invention, sensing ofventricular events on either channel will trigger execution of statemachine in states 1 and 4. During states 2 and 3, R-wave sensingcontinues but state machine is triggered every predetermined number ofseconds, as described below.

Returning to FIG. 7A, a buffer of the most recent 12 R-R intervalsobtained during R-wave processing using the sensing scheme of FIG. 8,described above, for example, is independently maintained for each ofthe two sensing channels ECG1 and ECG2. When the next sensed R-wave isobtained, Block 322, which initially would be the 12^(th) R-waveinterval, a heart rate estimate is determined, Block 323, using a metricof heart rate, such as the mean, trimmed mean, or median of the RRintervals, for example. According to an embodiment of the presentinvention, the 9^(th) fastest beat of the 12 beats on a beat by beatbasis is utilized as the heart rate metric. Using the 9^(th) fastestbeat provides an estimate of heart rate that is less susceptible tooversensing while maintaining reasonable sensitivity to short R-Rintervals as in the case of VT/VF. If the buffer of 12 R-R intervalscontains any unknown R-R intervals (i.e., because the buffer is not yetfilled) the initial estimate of heart rate is unknown.

Once the heart rate estimate is obtained using the heart rate metric, adetermination is made as to whether asystole is detected for eitherchannel, ECG1 or ECG 2, Block 324. According to an embodiment of thepresent invention, asystole is detected for the channel, for example,either by determining whether one of the 12 R-R intervals is greaterthan a predetermined time period, such as three seconds, for example, orif the time since the most recently sensed R wave exceeds apredetermined time period, such as three seconds, for example. Thelatter can occur if an R-wave is sensed, for example, in one channelECG1, but the other channel ECG2 has not had an R-wave sense in three ormore seconds. If asystole is detected for either of the two channelsECG1 or ECG2, the current 12 R-R intervals for channels that aredetermined to be in asystole are cleared from the buffers, Block 325,and the process continues by determining whether the current heart rateestimate is reliable for both channels ECG1 and ECG2, Block 328,described below.

If asystole is not detected for either channel ECG1 and ECG2, NO inBlock 324, a determination is made independently for both channels ECG1and ECG2 as to whether the current heart rate estimate for both channelsECG1 and ECG2 is reliable, Block 328. According to an embodiment of thepresent invention, the current heart rate estimate for each of the twochannels ECG1 and ECG2 is determined not to be reliable, No in Block328, if either there are unknown or cleared entries in the buffer forthat channel, or if a predetermined number of the sensed R-wavesassociated with the current 12 R-R intervals for that channel was sensedat the minimum sensing threshold level, i.e., the first threshold level403 of FIG. 8, for example, and if the current heart rate estimate forthe channel is less than the programmed heart rate threshold. Accordingto one embodiment, the predetermined number of sensed R-waves that mustbe sensed at the minimum threshold is set at two, for example. Inaddition, the programmed heart rate threshold may be within a range of150 to 240 beats per minute, and is nominally set at 180 beats perminute, for example. It is understood that while the processing isdescribed using a buffer of 12 R-R intervals, any number of intervalsand predetermined number of sensed R-waves that must be sensed at theminimum threshold may be utilized.

If the above analysis does not determine that both of the channels arereliable, No in Block 328, a determination is made as to whether justone of the channels was unreliable or if both channels were unreliable,Block 330. If both channels are determined to be unreliable, the current12 R-R intervals for both channels ECG1 and ECG2 are cleared from thebuffers, Block 326, and the next R-sense is obtained for each channel,Block 322 using the sensing scheme of FIG. 8, described above, so that anew heart rate estimate is determined, Block 323, based on the new R-Rintervals.

If only one channel is determined to unreliable, the value for the heartrate estimate for both channels is set to the current heart rateestimate for the channel determined to be reliable, Block 332. Onceeither both channels are determined to be reliable, YES in Block 328, oronly one of the two channels is determined to be unreliable andtherefore the heart rate estimate for both channels is set to thecurrent heart rate estimate for the channel determined to be reliable,Block 332, the final heart rate estimate is determined for each channelECG1 and ECG2 based on those results, Block 334, i.e., the heart rateestimate for each channel is set equal to their respective heart rateestimates determined in Block 323, or both are set equal to the heartrate estimate associated with the channel determined to be reliable,Block 332. A determination is then made as to whether the final heartrate estimates for both channels is greater than a predetermined VT/VFthreshold, Block 336. According to an embodiment of the presentinvention, the predetermined VT/VF threshold of Block 336 is set at 180bpm, for example, although any desired threshold could be utilized.

If the final heart rate estimates for one or both channels is notgreater than the predetermined VT/VF threshold, the buffer containingthe 12 R-R intervals for the channel not greater than the predeterminedVT/VF threshold is updated by removing the first R-sense, shifting theremaining eleven R-sense samples back so that the second R-sense becomesthe first R-sense, and so forth, and inserting the next detectedR-sense, Block 322, as the twelfth R-sense for each correspondingchannel ECG1 and ECG2. A new current heart rate estimate is thendetermined, Block 323. Once the final heart rate estimates for bothchannels is greater than the predetermined VT/VF threshold, Yes in Block336, the process transitions from the not concerned state 302 to theconcerned state 304.

According to the present invention, upon transition from the notconcerned state 302 to the concerned state 304, Block 305, a most recentwindow of ECG data from both channels ECG1 and ECG2 are utilized, suchas three seconds, for example, so that processing is triggered in theconcerned state 304 by a three-second timeout, rather than by thesensing of an R-wave, which is utilized when in the not concerned state302, described above. It is understood that while the processing isdescribed as being triggered over a three second period, other timesperiods for the processing time utilized when in the concerned state 304may be chosen, but should preferably be within a range of 0.5 to 10seconds. As a result, although sensing of individual R-waves continuesto occur in both channels ECG1 and ECG2 when in the concerned state 304,and the buffer of 12 R-R intervals continues to be updated, theopportunities for changing from the concerned state 304 to another stateand the estimates of heart rate only occur once the three-second timerexpires. Upon initial entry to the concerned state 304, it isadvantageous to process the most recent three-seconds of ECG data, i.e.,ECG data for the three seconds leading up to the transition to theconcerned state 304. This requires a continuous circular buffering ofthe most recent three seconds of ECG data even while in the notconcerned state 302.

As described in detail below, while in the concerned state 304, thepresent invention determines how sinusoidal and how noisy the signalsare in order to determine the likelihood that a ventricular fibrillation(VF) or fast ventricular tachycardia (VT) event is taking place, sincethe more sinusoidal and low noise the signal is, the more likely a VT/VFevent is taking place. As illustrated in FIG. 7B, once the devicetransitions from the not concerned state 302 to the concerned state 304,Block 305, a buffer for each of the two channels ECG1 and ECG2 forstoring classifications of 3-second segments of data as “shockable” or“non-shockable” is cleared. Processing of signals of the two channelsECG1 and ECG2 while in the concerned state 304 is then triggered by thethree second time period, rather than by the sensing of an R-waveutilized during the not concerned state 302, described above.

Once the three second time interval has expired, YES in Block 341,morphology characteristics of the signal during the three second timeinterval for each channel are utilized to determine whether the signalsare likely corrupted by noise artifacts and to characterize themorphology of the signal as “shockable” or “not shockable”. For example,using the signals associated with the three second time interval, adetermination is made for each channel ECG1 and ECG 2 as to whether thechannel is likely corrupted by noise, Block 342, and a determination isthen made as to whether both channels ECG1 and ECG2 are corrupted bynoise, Block 344.

As illustrated in FIG. 7C, the determination as to whether the signalassociated with each of the channels ECG1 and ECG2 is likely corruptedby noise, Block 342 of FIG. 7B, includes multiple sequential noise teststhat are performed on each channel ECG and ECG2. During a first noisetest, for example, a determination is made as to whether a metric ofsignal energy content of the signal for the channel is withinpredetermined limits, Block 380. For example, the amplitude of eachsample associated with the three second window is determined, resultingin N sample amplitudes, from which a mean rectified amplitude iscalculated as the ratio of the sum of the rectified sample amplitudes tothe total number of sample amplitudes N for the segment. If the samplingrate is 256 samples per second, for example, the total number of sampleamplitudes N for the three-second segment would be N=768 samples.

Once the mean rectified amplitude is calculated, a determination is madeas to whether the mean rectified amplitude is between an upper averageamplitude limit and a lower average amplitude limit, the lower averageamplitude limit being associated with asystole episodes without artifactand the upper average amplitude limit being associated with a valuegreater than what would be associated with ventricular tachycardia andventricular fibrillation events. According to an embodiment of thepresent invention, the upper average amplitude limit is set as 1.5 mV,and the lower average amplitude limit is set as 0.013 mV. While themetric of signal energy content is described above as the mean rectifiedamplitude, it is understood that other signal of energy contents couldbe utilized.

If the determined mean rectified amplitude is not between the upperaverage amplitude limit and the lower average amplitude limit, the threesecond segment for that channel is identified as being likely corruptedwith noise, Block 386, and no further noise tests are initiated for thatchannel's segment.

If the determined mean rectified amplitude is located between the upperaverage amplitude limit and the lower average amplitude limit, a noiseto signal ratio is calculated and a determination is made as to whetherthe noise to signal ratio is less than a predetermined noise to signalthreshold, Block 382. For example, the amplitude of each sampleassociated with the three second window is determined, resulting in Nraw sample amplitudes. The raw signal is lowpass filtered, resulting inL lowpass sample amplitudes. The raw mean rectified amplitude isdetermined as the average of the absolute values of the raw sampleamplitudes. The lowpass mean rectified amplitude is determined as theaverage of the absolute values of the lowpass sample amplitudes. Next, ahighpass mean rectified amplitude is then calculated as the differencebetween the raw mean rectified amplitude and the lowpass mean rectifiedamplitude. The noise to signal ratio is then determined as the ratio ofthe highpass mean rectified amplitude to the lowpass mean rectifiedamplitude. If the noise to signal ratio is greater than a predeterminedthreshold, such as 0.0703, for example, the three second segment forthat channel is identified as being likely corrupted with noise, Block386, and no further noise tests are initiated for the segment.

If the noise to signal ratio is less than or equal to the predeterminedthreshold, a determination is made as to whether the signal is corruptedby muscle noise, Block 384. According to an embodiment of the presentinvention, the determination as to whether the signal is corrupted bymuscle noise is made by determining whether the signal includes apredetermined number of signal inflections indicative of the likelihoodof the signal being corrupted by muscle noise, using a muscle noisepulse count that is calculated to quantify the number of signalinflections in the three second interval for each channel ECG1 and ECG2.The presence of a significant number of inflections is likely indicativeof muscle noise.

FIG. 9A is a graphical representation of a determination of whether asignal is corrupted by muscle noise according to an embodiment of thepresent invention. FIG. 9B is a flowchart of a method of determiningwhether a signal is corrupted by muscle noise according to an embodimentof the present invention. For example, as illustrated in FIGS. 9A and9B, in order to determine a muscle noise count for the three secondinterval, the raw signal 420 is applied to a first order derivativefilter to obtain a derivative signal 422, and all of the zero-crossings424 in the derivative signal 422 are located, Block 460. A data paircorresponding to the data points immediately prior to and subsequent tothe zero crossings 424, points 426 and 428 respectively, for eachcrossing is obtained. The value of the data point in each data pair withsmaller absolute value is zeroed in order to allow a clear demarcationof each pulse when a rectified signal 430 is derived from the derivativesignal 422 with zeroed zero-crossing points 432.

A pulse amplitude threshold Td, for determining whether the identifiedinflection is of a significant amplitude to be identified as beingassociated with muscle noise, is determined, Block 462, by dividing therectified signal from the three second segment into equal sub-segments434, estimating a local maximum amplitude 436-442 for each of thesub-segments 434, and determining whether the local amplitudes 436-442are less than a portion of the maximum amplitude, which is maximumamplitude 440 in the example of FIG. 9, for the whole three secondsegment. If the local maximum amplitude is less than the portion of themaximum amplitude for the whole three second segment, the local maximumamplitude is replaced by the maximum for the whole three second segmentfor the sub-segment corresponding to that local maximum amplitude.

It is understood that while only two or less zero-crossing points areshown as being located within the sub-segments in the illustration ofFIG. 9 for the sake of simplicity, in fact each of the sub-segments 434,which have a length of approximately 750 milliseconds, will contain manyinflections, such as every 25 milliseconds, for example.

According to an embodiment of the present invention, the three secondsegment is divided into four sub-segments and the local maximumamplitudes are replaced by the maximum amplitude for the whole segmentif the local maximum amplitude is less than one fifth of the maximumamplitude for the whole segment. Once the determination of whether toreplace the local maximum amplitudes for each of the sub-segments withthe maximum amplitude for the whole segment is completed, the pulseamplitude threshold Td for the segment is set equal to a predeterminedfraction of the mean of the local maximum amplitudes for each of thesub-segments. According to an embodiment of the present invention, thepulse amplitude threshold Td for the three second segment is set equalto one sixth of the mean of the local maximum amplitudes 436-440.

Once the pulse amplitude threshold Td has been determined, theinflections associated with the signal for the three second segment isclassified as being of significant level to be likely indicative ofnoise by determining whether the pulse amplitude threshold Td is lessthan a pulse threshold, Block 464. According to an embodiment of thepresent invention, the pulse threshold is set as 1 microvolt. If thepulse amplitude threshold Td is less than the pulse threshold, thesignal strength is too small for a determination of muscle noise, andtherefore the signal is determined to be not likely corrupted by noiseand therefore the channel is determined to be not noise corrupted, Block466.

If the pulse amplitude threshold Td is greater than or equal to thepulse threshold, the three second segment is divided into twelvesub-segments of 250 ms window length, the number of muscle noise pulsesin each sub-segment is counted, and both the sub-segment having themaximum number of muscle noise pulses and the number of sub-segmentshaving 6 or more muscle noise pulses that are greater than apredetermined minimum threshold is determined. Muscle noise isdetermined to be present in the signal if either the maximum number ofmuscle noise pulses in a single sub-segment is greater than a noisepulse number threshold or the number of sub-segments of the twelvesub-segments having 6 or more muscle noise pulses greater than theminimum threshold is greater than or equal to a sub-segment pulse countthreshold. According to an embodiment of the present invention, thenoise pulse number threshold is set equal to eight and the sub-segmentpulse count threshold is set equal to three.

For example, if the pulse amplitude threshold Td is greater than orequal to the pulse threshold, No in Block 464, the maximum number ofmuscle noise counts in a single sub-segment is determined, Block 468. Ifthe maximum number of muscle noise counts is greater than the noisepulse number threshold, Yes in Block 470, the channel is determined tobe noise corrupted, Block 472. If the maximum number of muscle noisecounts for the channel is less than or equal to the noise pulse numberthreshold, No in Block 470, the number of sub-segments of the twelvesub-segments having 6 or more muscle noise pulses greater than theminimum threshold is determined, Block 474, and if the number is greaterthan or equal to a sub-segment pulse count threshold, Yes in Block 476,the channel is determined to be noise corrupted, Block 472. If thenumber is less than the sub-segment pulse count threshold, No in Block476, the channel is determined not to be noise corrupted, Block 466.

FIG. 9C is a flowchart of a method of determining whether a signal iscorrupted by muscle noise according to an embodiment of the presentinvention. Since muscle noise can be present during an episode ofventricular tachycardia, the width of the overall signal pulse waveformis determined in order to distinguish between signals that aredetermined likely to be purely noise related and signals that are bothshockable events and determined to include noise. Therefore, asillustrated in FIG. 9C, according to an embodiment of the presentinvention, once muscle noise is determined to be present as a result ofthe muscle noise pulse count being satisfied, No in Block 470 and Yes inBlock 476, a determination is made as to whether the signal is bothnoise corrupted and shockable, Block 480.

According to an embodiment of the present invention, the determinationin Block 480 as to whether the signal is both noisy and shockable ismade, for example, by dividing the rectified signal, having 768 datapoints, into four sub-segments and determining a maximum amplitude foreach of the four sub-segments by determining whether a maximum amplitudefor the sub-segment is less than a portion of the maximum amplitude forthe entire rectified signal in the three second segment. For example, adetermination is made for each sub-segment as to whether the maximumamplitude for the sub-segment is less than one fourth of the maximumamplitude for the entire rectified signal. If less than a portion of themaximum amplitude for the entire rectified signal in the three secondsegment, the maximum amplitude for the sub-segment is set equal to themaximum amplitude for the entire rectified signal.

A mean rectified amplitude for each of the sub-segments is determined bydividing the sum of the rectified amplitudes for the sub-segment by thenumber of samples in the sub-segment, i.e., 768÷4. Then the normalizedmean rectified amplitude for each sub-segment is determined by dividingthe mean rectified amplitude for each of the sub-segments by the peakamplitude for the sub-segment. The normalized mean rectified amplitudefor the three second segment is then determined as the sum of thenormalized mean rectified amplitudes for each sub-segment divided by thenumber of sub-segments, i.e., four.

Therefore, once muscle noise is suspected as a result of thedetermination of the muscle noise pulse count, the determination ofBlock 480 based on whether the normalized mean rectified amplitude forthe three second segment is greater than a predetermined threshold foridentifying signals that, despite being indicative of a likelihood ofbeing associated with noise, nevertheless are associated with ashockable event. For example, according to an embodiment of the presentinvention, a determination is made as to whether the normalized meanrectified amplitude for the three second segment is greater than 18microvolts. If the normalized mean rectified amplitude for the threesecond segment is less than or equal to the predetermined threshold, thechannel is likely corrupted by muscle noise and not shockable, No inBlock 480, and is therefore identified as being corrupted by noise,Block 472. If the normalized mean rectified amplitude for the threesecond segment is greater than the predetermined threshold, the channelis determined to be likely corrupted by muscle noise and shockable, Yesin Block 480, and is therefore identified as not to be likely corruptedby muscle noise, Block 478.

Returning to FIG. 7C, when the signal is determined to be not likelycorrupted by muscle noise, a determination is made as to whether themean frequency of the signal associated with the channel is less than apredetermined mean frequency threshold, Block 388, such as 11 Hz forexample. The mean frequency of the signal during the 3 second segmentfor each channel ECG 1 and ECG2 is generated, for example, bycalculating the ratio of the mean absolute amplitude of the firstderivative of the 3 second segment to the mean absolute amplitude of the3 second segment, multiplied by a constant scaling factor. If the meanfrequency is determined to be greater than or equal to the predeterminedmean frequency threshold, No in Block 388, the three second segment forthat channel is identified as being likely corrupted with noise, Block386. If the mean frequency is determined to be less than thepredetermined mean frequency threshold, Yes in Block 388, the threesecond segment for that channel is identified as being not noisecorrupted, Block 390.

According to an embodiment of the present invention, since the meanspectral frequency tends to be low for true ventricular fibrillationevents, moderate for organized rhythms such as sinus rhythm andsupraventricular tachycardia, for example, and high during asystole andnoise, the determination in Block 388 includes determining whether themean frequency is less than a predetermined upper mean frequencythreshold, such as 11 Hz (i.e., mean period T of approximately 91milliseconds) for example, and whether the mean frequency is less than apredetermined lower mean frequency, such as 3 Hz for example. If themean frequency is below a second, lower threshold, such as 3 Hz, forexample, the signal is also rejected as noise and no further noise testsare initiated. This comparison of the mean frequency to a second lowerthreshold is intended to identify instances of oversensing, resulting inappropriate transition to the concerned state. If the mean frequency ofthe signal is less than 3 Hz, it is generally not possible for the heartrate to be greater than 180 beats per minute. In practice, it may beadvantageous to set the lower frequency threshold equal to theprogrammed VT/VF detection rate, which is typically approximately 3 Hz.

Therefore, in the determination of Block 388, if the mean frequency isdetermined to be either greater than or equal to the predetermined uppermean frequency threshold or less than the lower threshold, the threesecond segment for that channel is identified as being likely corruptedwith noise, Block 386. If the mean frequency is determined to be bothless than the predetermined upper mean frequency threshold and greaterthan the lower threshold, the three second segment for that channel isidentified as not being noise corrupted, Block 390.

Returning to FIG. 7B, once the determination as to whether the channelsECG1 and ECG2 are corrupted by noise is made, Block 342, a determinationis made as to whether both channels are determined to be noisecorrupted, Block 344. If the signal associated with both channels ECG1and ECG2 is determined to likely be corrupted by noise, both channelsare classified as being not shockable, Block 347, and therefore a bufferfor each channel ECG1 and ECG 2 containing the last threeclassifications of the channel is updated accordingly. If both channelsECG1 and ECG2 are not determined to be likely corrupted by noise, No inBlock 344, the device distinguishes between either one of the channelsbeing not corrupted by noise or both channels being not corrupted bynoise by determining whether noise was determined to be likely in onlyone of the two channels ECG1 and ECG2, Block 346.

If noise was likely in only one of the two channels, a determination ismade whether the signal for the channel not corrupted by noise, i.e.,the clean channel, is more likely associated with a VT event or with aVF event by determining, for example, whether the signal for thatchannel includes R-R intervals that are regular and the channel can betherefore classified as being relatively stable, Block 348. If the R-Rintervals are determined not to be relatively stable, NO in Block 348,the signal for that channel is identified as likely being associatedwith VF, which is then verified by determining whether the signal is ina VF shock zone, Block 350, described below. If R-R intervals for thatchannel are determined to be stable, YES in Block 348, the signal isidentified as likely being associated with VT, which is then verified bydetermining whether the signal is in a VT shock zone, Block 352,described below.

If noise was not likely for both of the channels, No in Block 346, i.e.,both channels are determined to be clean channels, a determination ismade whether the signal for both channels is more likely associated witha VT event or with a VF event by determining whether the signal for bothchannels includes R-R intervals that are regular and can be thereforeclassified as being relatively stable, Block 356. If the R-R intervalsare determined not to be relatively stable, NO in Block 356, the signalfor both channels is identified as likely being associated with VF,which is then verified by determining whether the signal for eachchannel is in a VF shock zone, Block 360, described below. If R-Rintervals for both channels are determined to be stable, YES in Block356, the signal is identified as likely being associated with VT, whichis then verified by determining, based on both channels, whether thesignal is in a VT shock zone, Block 352.

As illustrated in FIG. 7D, according to an embodiment of the presentinvention, in order to determine whether the signal for both channelsincludes R-R intervals that are regular and the channels can betherefore classified as being relatively stable, Block 356,predetermined maximum and minimum intervals for each channel ECG1 andECG2 are identified, Block 500, from the updated buffer of 12RR-intervals, Block 342. According to one embodiment of the presentinvention, the largest RR-interval and the sixth largest RR-interval ofthe twelve RR-intervals are utilized as the maximum interval and theminimum interval, respectively.

The difference between the maximum RR-interval and the minimumRR-interval of the 12 RR-intervals is calculated for each channel ECG1and ECG2, Block 502, to generate a first interval difference associatedwith the first channel ECG1 and a second interval difference associatedwith the second channel ECG2. The smallest of the first intervaldifference and the second interval difference is then identified, Block504, and a determination is made as to whether the minimum of the firstinterval difference and the second interval difference is greater than apredetermined stability threshold, Block 506, such as 110 milliseconds,for example.

If the minimum of the first interval difference and the second intervaldifference is greater than the stability threshold, the event isclassified as an unstable event, Block 508, and a determination is madefor each channel as to whether the signal associated with the channel iswithin a predetermined VF shock zone, Blocks 360 and 362 of FIG. 7B,described below. If the minimum of the first interval difference and thesecond interval difference is less than or equal to the stabilitythreshold, No in Block 506, the device determines which one of theminimum RR-interval associated with the first channel ECG1 and theminimum RR-interval associated with the second channel ECG2 is shortest,Block 510, and determines whether the shortest minimum interval isgreater than a minimum interval threshold, Block 512, such as 200milliseconds, for example.

If the shortest of the two minimum intervals is less than or equal tothe minimum interval threshold, the event is classified as an unstableevent, Block 508, and a determination is made for each channel as towhether the signal associated with the channel is within a predeterminedVF shock zone, Blocks 360 and 362 of FIG. 7B, described below. If theshortest of the two minimum intervals is greater than the minimuminterval threshold, the device determines which one of the minimumRR-interval associated with the first channel ECG1 and the minimumRR-interval associated with the second channel ECG2 is the greatest,Block 514, and determines whether the maximum of the two minimumintervals is less than or equal to a maximum interval threshold, Block516, such as 333 milliseconds for example. If the maximum of the twominimum intervals is greater than the maximum interval threshold, theevent is classified as an unstable event, Block 508, and a determinationis made for each channel as to whether the signal associated with thechannel is within a predetermined VF shock zone, Blocks 360 and 362 ofFIG. 7B, described below. If the maximum of the two minimum intervals isless than or equal to the maximum interval threshold, the event isclassified as a stable event, Block 518, and a determination is made,based on both channels ECG1 and ECG2, as to whether the signal is withina predetermined VT shock zone, Block 358, described below.

Returning to FIG. 7B, according to an embodiment of the presentinvention, during the determination of whether the signal associatedwith each of the channels ECG1 and ECG2 is within the VF shock zone,Blocks 360 and 362, the VF shock zone is defined based upon a low slopecontent metric and a spectral width metric for each of the two channelsECG1 and ECG2. The low slope content metric is calculated as the ratioof the number of data points with low slope to the total number ofsamples in the 3-second segment. For example, according to an embodimentof the present invention, the difference between successive ECG samplesis determined as an approximation of the first derivative (i.e., theslope) of the ECG signal. In particular, as illustrated in FIG. 7E, theraw signal for each channel is applied to a first order derivativefilter to obtain a derivative signal for the three-second segment, Block530. The derivative signal is then rectified, divided into four equalsub-segments, and the largest absolute slope is estimated for each ofthe four sub-segments, Block 532.

A determination is made as to whether the largest absolute slopes areless than a portion of the overall largest absolute slope for the wholethree-second segment, Block 534, such as one-fifth of the overallabsolute slope, for example. If the largest absolute slope is less thanthe portion of the overall slope, then the slope value for thatsub-segment is set equal to the overall largest absolute slope, Block536. If the largest absolute slope is not less than the portion of theoverall slope, then the slope value for that sub-segment is set equal tothe determined largest absolute slope for the sub-segment, Block 538.

Once the slope value for each of the sub-segments has been determinedand updated by being set equal to the largest slope for the three secondsegment, if necessary, the average of the four slopes is calculated anddivided by a predetermined factor, such as 16 for example, to obtain alow slope threshold, Block 540. The low slope content is then obtainedby determining the number of sample points in the three-second segmenthaving an absolute slope less than or equal to the low slope threshold,Block 542.

According to an embodiment of the present invention, if, during thedetermination of the low slope threshold in Block 540, the low slopethreshold is a fraction, rather than a whole number, a correction ismade to the low slope content to add a corresponding fraction of thesamples. For example, if the threshold is determined to be 4.5, then thelow slope content is the number of sample points having an absoluteslope less than or equal to 4 plus one half of the number of samplepoints with slope equal to 5.

The spectral width metric, which corresponds to an estimate of thespectral width of the signal for the three-second segment associatedwith each channel ECG1 and ECG2, is defined, for example, as thedifference between the mean frequency and the fundamental frequency ofthe signal. According to an embodiment of the present invention, thespectral width metric is calculated by determining the differencebetween the most recent estimate of the RR-cycle length and the meanspectral period of the signal for that channel. As is known in the art,the mean spectral period is the inverse of the mean spectral frequency.

It is understood that R-R cycle length utilized in the concerned stateand armed state can be different than that used in the not concernedstate. For example, according to an embodiment of the present invention,the 9^(th) longest R-R interval is utilized in the not concerned stateand the mean of the 7^(th) to the 10^(th) R-R interval is utilized inthe concerned state and the armed state.

FIG. 10 is a graphical representation of a VF shock zone according to anembodiment of the present invention. As illustrated in FIG. 10, a VFshock zone 500 is defined for each channel ECG1 and ECG2 based on therelationship between the calculated low slope content and the spectralwidth associated with the channel. For example, the shock zone isdefined by a first boundary 502 associated with the low slope contentset for by the equation:Low slope content=−0.0013×spectral width+0.415  Equation 1and a second boundary 504 associated with the spectral width set forthby the equation:spectral width=200  Equation 2

As can be seen in FIG. 10, since noise 506 tends to have a relativelyhigher spectral width, and normal sinus rhythm 508 tends to have arelatively higher low slope content relative to VF, both noise 506 andnormal sinus rhythm 508 would be located outside the VF shock zone 500.

A determination is made for each channel ECG1 and ECG2 as to whether thelow slope content for that channel is less than both the first boundary502 and the spectral width is less than the second boundary 504, i.e.,the low slope content is less than −0.0013×spectral width+0.415, and thespectral width is less than 200. For example, once the event isdetermined to be associated with VF, i.e., the intervals for bothchannels are determined to be irregular, No in Block 356, adetermination is made that channel ECG1 is in the VF shock zone, Yes inBlock 360, if, for channel ECG1, both the low slope content is less thanthe first boundary 502 and the spectral width is less than the secondboundary 504. The three second segment for that channel ECG1 is thendetermined to be shockable, Block 363 and the associated buffer for thatchannel is updated accordingly. If either the low slope content for thechannel is not less than the first boundary 502 or the spectral width isnot less than the second boundary, the channel ECG1 is determined not tobe in the VF shock zone, No in Block 360, the three second segment forthat channel ECG1 is then determined to be not shockable, Block 365, andthe associated buffer is updated accordingly.

Similarly, a determination is made that channel ECG2 is in the VF shockzone, Yes in Block 362, if, for channel ECG2, both the low slope contentis less than the first boundary 502 and the spectral width is less thanthe second boundary 504. The three second segment for that channel ECG2is then determined to be shockable, Block 369 and the associated bufferfor that channel is updated accordingly. If either the low slope contentfor the channel is not less than the first boundary 502 or the spectralwidth is not less than the second boundary, the channel ECG2 isdetermined not to be in the VF shock zone, No in Block 362, the threesecond segment for that channel ECG2 is then determined to be notshockable, Block 367, and the associated buffer is updated accordingly.

According to an embodiment of the present invention, rather than beingdefined by Equation 1, the shock zone may be defined so that the firstboundary 502 associated with the low slope content is set forth by thefollowing equation:Low slope content=−0.005×spectral width+1.1  Equation 1Aso that the VF shock zone 500 is defined, as described above, usingEquation 1A and Equation 2.

FIGS. 11A and 11B are graphical representations of the determination ofwhether an event is within a shock zone according to an embodiment ofthe present invention. During the determination of whether the event iswithin the VT shock zone, Block 358 of FIG. 7B, the low slope contentand the spectral width is determined for each channel ECG1 and ECG2, asdescribed above in reference to determining the VF shock zone. Adetermination is made as to which channel of the two signal channelsECG1 and ECG2 contains the minimum low slope content and which channelof the two signal channels ECG 1 and ECG2 contains the minimum spectralwidth. A first VT shock zone 520 is defined based on the relationshipbetween the low slope content associated with the channel determined tohave the minimum low slope content and the spectral width associatedwith the channel determined to have the minimum spectral width. Forexample, according to an embodiment of the present invention, the firstVT shock zone 520 is defined by a boundary 522 associated with theminimum low slope content and the minimum spectral width set forth bythe equation:LSC=−0.004×SW+0.93  Equation 3

A second VT shock zone 524 is defined based on the relationship betweenthe low slope content associated with the channel determined to have theminimum low slope content and the normalized mean rectified amplitudeassociated with the channel determined to have the maximum normalizedmean rectified amplitude. The normalized mean rectified amplitudes forthe two channels ECG1 and ECG2 utilized during the VT shock zone test isthe same as described above in reference to the noise determination ofBlock 343. For example, according to an embodiment of the presentinvention, the second VT shock zone 524 is defined by a second boundary526 associated with the relationship between the minimum low slope countand the maximum normalized mean rectified amplitude set forth by theequation:NMRA=68×LSC+8.16  Equation 4

If both the minimum low slope count is less than the first boundary 522,i.e., −0.004×minimum spectral width+0.93, and the maximum normalizedmean rectified amplitude is greater than the second boundary 526, i.e.,68×minimum low slope count+8.16, the event is determined to be in the VTshock zone, YES in Block 358, and both channels ECG1 and ECG2 aredetermined to be shockable, Block 357, and the associated buffers areupdated accordingly. If either the minimum low slope count is not lessthan the first boundary 522 or the maximum normalized mean rectifiedamplitude is not greater than the second boundary 526, the event isdetermined to be outside the VT shock zone, NO in Block 358, and bothchannels ECG1 and ECG2 are determined to be not shockable, Block 359.

As described, during both the VF shock zone test, Blocks 360 and 362,and the VT shock zone test, Block 358, the test results for each channelECG1 and ECG2 as being classified as shockable or not shockable arestored in a rolling buffer containing the most recent eight suchdesignations, for example, for each of the two channels ECG1 and ECG2that is utilized in the determination of Block 356, as described below.

If only one of the two channels ECG1 and ECG2 is determined to becorrupted by noise, Yes in Block 346, a determination is made whetherthe signal for the channel not corrupted by noise, i.e., the “cleanchannel”, is more likely associated with a VT event or with a VF eventby determining whether the signal for the clean channel includes R-Rintervals that are regular and can be therefore classified as beingrelatively stable, Block 348. If the R-R intervals are determined not tobe relatively stable, NO in Block 348, the signal for the clean channelis identified as likely being associated with VF, which is then verifiedby determining whether the signal for the clean channel is in a VF shockzone, Block 350, described below. If R-R intervals for the clean channelare determined to be stable, YES in Block 348, the signal is identifiedas likely being associated with VT, which is then verified bydetermining whether the signal for the clean channel is in a VT shockzone, Block 352.

According to an embodiment of the present invention, in order todetermine whether the signal for the clean channel includes R-Rintervals that are regular and the clean channel can be thereforeclassified as being either relatively stable, Yes in Block 348, orrelatively unstable, No in Block 348, the device discriminates VT eventsfrom VF events in Block 348 by determining whether the relative level ofvariation in the RR-intervals associated with the clean channel isregular. For example, as illustrated in FIG. 7H, predetermined maximumand minimum intervals for the clean channel are identified, Block 700,from the updated buffer of 12 RR-intervals, Block 342 of FIG. 7B.According to an embodiment of the present invention, the largestRR-interval and the sixth largest RR-interval of the twelve RR-intervalsare utilized as the maximum interval and the minimum interval,respectively.

The difference between the maximum RR-interval and the minimumRR-interval of the 12 RR-intervals is calculated to generate an intervaldifference associated with the clean channel, 702. A determination isthen made as to whether the interval difference is greater than apredetermined stability threshold, Block 704, such as 110 milliseconds,for example.

If the interval difference is greater than the stability threshold, theevent is classified as an unstable event, Block 706, and therefore theclean channel is determined not to include regular intervals, No inBlock 348, and a determination is made as to whether the signalassociated with the clean channel is within a predetermined VF shockzone, Block 350 of FIG. 7B, described below. If the interval differenceis less than or equal to the stability threshold, No in Block 704, thedevice determines whether the minimum RR interval is greater than aminimum interval threshold, Block 710, such as 200 milliseconds, forexample.

If the minimum interval is less than or equal to the minimum intervalthreshold, No in Block 710, the event is classified as an unstableevent, Block 706, and therefore the clean channel is determined not toinclude regular intervals, No in Block 348, and a determination is madeas to whether the signal associated with the clean channel is within apredetermined VF shock zone, Block 350 of FIG. 7B, described below. Ifthe minimum interval is greater than the minimum interval threshold, Yesin Block 710, the device determines whether the maximum interval is lessthan or equal to a maximum interval threshold, Block 712, such as 333milliseconds for example. If the maximum interval is greater than themaximum interval threshold, the event is classified as an unstableevent, Block 706, and therefore the clean channel is determined not toinclude regular intervals, No in Block 348, and a determination is madeas to whether the signal associated with the clean channel is within apredetermined VF shock zone, Block 350 of FIG. 7B, described below. Ifthe maximum interval is less than or equal to the maximum intervalthreshold, the event is classified as a stable event, Block 714, andtherefore the clean channel is determined to include regular intervals,Yes in Block 348, and a determination is made as to whether the signalassociated with the clean channel is within a predetermined VT shockzone, Block 352 of FIG. 7B, described below.

Returning to FIG. 7B, according to an embodiment of the presentinvention, the determination of whether the clean channel is within theVF shock zone, Block 350, is made based upon a low slope content metricand a spectral width metric, similar to the VF shock zone determinationdescribed above in reference to Blocks 360 and 362, both of which aredetermined for the clean channel using the method described above. Oncethe low slope content metric and a spectral width metric are determinedfor the clean channel, the determination of whether the clean channel isin the VF shock zone is made using Equations 1 and 2, so that if eitherthe low slope content for the clean channel is not less than the firstboundary 502 or the spectral width is not less than the second boundary504, the clean channel is determined not to be in the VF zone, No inBlock 350 and both channels are classified as not shockable, Block 351,and the associated buffers are updated accordingly.

If the low slope content for the clean channel is less than the firstboundary 502 and the spectral width is less than the second boundary504, the clean channel is determined to be in the VF zone, Yes in Block350. A determination is then made as to whether the channel determinedto be corrupted by noise, i.e., the “noisy channel”, is within the VFshock zone, Block 354. If either the low slope content for the noisychannel is not less than the first boundary 502 or the spectral width isnot less than the second boundary 504, the noisy channel is determinednot to be in the VF zone, No in Block 354, the clean channel isclassified as shockable and the noisy channel is classified as notshockable, Block 355, and the associated buffers are updatedaccordingly.

If the low slope content for the noisy channel is less than the firstboundary 502 and the spectral width is less than the second boundary504, the noisy channel is determined to be in the VF zone, Yes in Block354, both the clean channel and the noisy channel are classified asbeing shockable, Block 353, and the associated buffers are updatedaccordingly.

Similar to the VT shock zone determination described above in referenceto Block 358, during the determination as to whether the clean channelis within the VT shock zone in Block 352, the low slope content and thespectral width is determined for the clean channel as described above inreference to determining the VF shock zone. The first VT shock zone 520is defined based on the relationship between the low slope content andthe spectral width associated with the clean channel according toEquation 3, for example, and the second VT shock zone 524 is definedbased on the relationship between the low slope count and the normalizedmean rectified amplitude associated with the clean channel. Thenormalized mean rectified amplitudes for the clean channel is the sameas described above in reference to the noise detection tests of Block344. For example, according to an embodiment of the present invention,the second VT shock zone 524 is defined by a second boundary 526associated with the relationship between the low slope count and thenormalized mean rectified amplitude of the clean channel using Equation4.

If both the low slope count is less than the first boundary 522, i.e.,−0.004×spectral width of clean channel+0.93, and the normalized meanrectified amplitude is greater than the second boundary 526, i.e.,68×low slope count of clean channel+8.16, the clean channel isdetermined to be in the VT shock zone, Yes in Block 352, both channelsare classified as being shockable, Block 353, and the associated buffersare updated accordingly.

If either the low slope count is not less than the first boundary 522 orthe maximum normalized mean rectified amplitude is not greater than thesecond boundary 526, the clean channel is determined to be outside theVT shock zone, No in Block 352, both channels are classified as beingnot shockable, Block Block 351, and the associated buffers are updatedaccordingly.

FIG. 12 is a graphical representation of a shock zone according to anembodiment of the present invention. According to an embodiment of thepresent invention, during the determination of whether the signalassociated with each of the channels ECG1 and ECG2 is within the VFshock zone, Blocks 360 and 362, the VF shock zone is defined based upona normalized mean rectified amplitude metric and a spectral width metricfor each of the two channels ECG1 and ECG2, both of which may begenerated, for example, as described above. In particular, according tothe embodiment of FIG. 12, a VF shock zone 800 is defined for eachchannel ECG1 and ECG2, during a given three second sensing window, basedon the relationship between the calculated normalized mean rectifiedamplitude and the spectral width associated with the channel, with theshock zone 800 being defined by a boundary 802 associated with thenormalized mean rectified amplitude by the equation:Normalized Mean Rectified Amplitude=0.2×spectral width+3  Equation. 5

As can be seen in FIG. 12, since noise 806 tends to have a relativelyhigher spectral width, and normal sinus rhythm 808 tends to have arelatively lower normalized mean rectified amplitude relative to VF,both noise 806 and normal sinus rhythm 808 are located outside the VFshock zone 800. Therefore, a determination is made for each channel ECG1and ECG2, during a given three second sensing window, as to whether thenormalized mean rectified amplitude for that channel is greater than orequal to the boundary 802, i.e., the normalized mean rectified amplitudeis greater than or equal to 0.2×spectral width+3. For example, once theevent is determined to be associated with VF, i.e., the intervals forboth channels are determined to be irregular, No in Block 356, adetermination is made that channel ECG1 is in the VF shock zone, Yes inBlock 360, if, for channel ECG1, the normalized mean rectified amplitudeis greater than or equal to the boundary 802. The three second segmentfor that channel ECG1 is then determined to be shockable, Block 363 andthe associated buffer for that channel is updated accordingly. If thenormalized mean rectified amplitude for the channel is less than theboundary 802, the channel ECG1 is determined not to be in the VF shockzone, No in Block 360, the three second segment for that channel ECG1 isthen determined to be not shockable, Block 365, and the associatedbuffer is updated accordingly.

Similarly, a determination is made that channel ECG2 is in the VF shockzone, Yes in Block 362, if, for channel ECG2, the normalized meanrectified amplitude is greater than or equal to the boundary 802. Thethree second segment for that channel ECG2 is then determined to beshockable, Block 369 and the associated buffer for that channel isupdated accordingly. If the normalized mean rectified amplitude for thechannel is less than the boundary 802, the channel ECG2 is determinednot to be in the VF shock zone, No in Block 362, the three secondsegment for that channel ECG2 is then determined to be not shockable,Block 367, and the associated buffer is updated accordingly.

FIG. 13 is a graphical representation of the determination of whether anevent is within a shock zone according to an embodiment of the presentinvention. According to another embodiment of the present invention,during the determination of whether the event is within the VT shockzone, Block 358 of FIG. 7B, the normalized mean rectified amplitude andthe spectral width are determined for each channel ECG1 and ECG2 duringa given three second sensing window, as described above in reference todetermining the VF shock zone. A determination is made as to whichchannel of the two signal channels ECG1 and ECG2 contains the maximumnormalized mean rectified amplitude and which channel of the two signalchannels ECG 1 and ECG2 contains the minimum spectral width. A VT shockzone 820 is defined based on the relationship between the normalizedmean rectified amplitude associated with the channel determined to havethe maximum normalized mean rectified amplitude and the spectral widthassociated with the channel determined to have the minimum spectralwidth. For example, according to an embodiment of the present invention,the VT shock zone 820 is defined, for a given three second sensingwindow, by a boundary 822 associated with the maximum normalized meanrectified amplitude and the minimum spectral width set forth by theequation:NMRA=0.3636×SW−15  Equation 6

If the maximum normalized mean rectified amplitude is greater than orequal to the boundary 822, i.e., 0.3636×minimum spectral width−15, theevent is determined to be in the VT shock zone, YES in Block 358, andthe three second segment for both channels ECG1 and ECG2 are determinedto be shockable, Block 357, and the associated buffers are updatedaccordingly. If the maximum normalized mean rectified amplitude is lessthan the boundary 822, the event is determined to be outside the VTshock zone, NO in Block 358, and both channels ECG1 and ECG2 aredetermined to be not shockable, Block 359.

Returning to FIG. 7B, according to another embodiment of the presentinvention, the determination of whether the clean channel is within theVF shock zone, Block 350, is made based upon a normalized mean rectifiedamplitude metric and a spectral width metric, similar to the VF shockzone determination described above in reference to Blocks 360 and 362,both of which are determined for the clean channel using the VF shockzone described above in reference to FIG. 12. In particular, once thenormalized mean rectified amplitude metric and a spectral width metricare determined for the clean channel, the determination of whether theclean channel is in the VF shock zone is made using Equation 5, so thatif the normalized mean rectified amplitude for the clean channel is lessthan the boundary 802, the clean channel is determined not to be in theVF zone, No in Block 350 and both channels are classified as notshockable, Block 351, and the associated buffers are updatedaccordingly.

If the normalized mean rectified amplitude for the clean channel isgreater than or equal to the boundary 802, the clean channel isdetermined to be in the VF zone, Yes in Block 350. A determination isthen made as to whether the channel determined to be corrupted by noise,i.e., the “noisy channel”, is within the VF shock zone, Block 354. Ifthe normalized mean rectified amplitude for the noisy channel is lessthan the boundary 802, the noisy channel is determined not to be in theVF zone, No in Block 354, the clean channel is classified as shockableand the noisy channel is classified as not shockable, Block 355, and theassociated buffers are updated accordingly.

If the normalized mean rectified amplitude for the noisy channel isgreater than or equal to the boundary 802, the noisy channel isdetermined to be in the VF zone, Yes in Block 354, both the cleanchannel and the noisy channel are classified as being shockable, Block353, and the associated buffers are updated accordingly.

Similar to the VT shock zone determination described above in Block 358using the VT shock zone of FIG. 13, during the determination as towhether the clean channel is within the VT shock zone in Block 352, thenormalized mean rectified amplitude and the spectral width aredetermined for the clean channel as described above in reference todetermining the VF shock zone. The VT shock zone 820 is then definedbased on the relationship between the normalized mean rectifiedamplitude and the spectral width associated with the clean channelaccording to Equation 6. If the maximum normalized mean rectifiedamplitude for the clean channel is greater than or equal to the boundary822, i.e., 0.3636×minimum spectral width−15, the clean channel isdetermined to be in the VT shock zone, YES in Block 352, and bothchannels ECG1 and ECG2 are determined to be shockable, Block 353, andthe associated buffers are updated accordingly. If the maximumnormalized mean rectified amplitude for the clean channel is less thanthe boundary 822, the clean channel is determined to be outside the VTshock zone, NO in Block 352, and both channels ECG1 and ECG2 aredetermined to be not shockable, Block 351.

Once the classification of both of the channels ECG1 and ECG2 is madesubsequent to the determination of whether the clean channel or channelsis in the VT shock zone, Block 352 and 358, or the VF shock zone, Blocks350 and Blocks 360 and 362 in combination, a determination is made as towhether the device should transition from the concerned state 304 to thearmed state 306, Block 370. For example, according to an embodiment ofthe present invention, the transition from the concerned state 304 tothe armed state 306 is confirmed if a predetermined number, such as twoout of three for example, of three-second segments for both channelsECG1 and ECG2 have been classified as being shockable. If thepredetermined number of three-second segments in both channels ECG1 andECG2 have been classified as shockable, the device transitions from theconcerned state 304 to the armed state 306, Yes in Block 370. If thepredetermined number of three-second segments in both channels ECG1 andECG2 have not been classified as shockable, the device does nottransition from the concerned state 304 to the armed state 306, no inBlock 370, and a determination as to whether to transition back to thenot concerned state 302 is made, Block 372. The determination as towhether to transition from the concerned state 304 back to the notconcerned state 302 is made, for example, by determining whether theheart rate estimate is less than a heart rate threshold level in both ofthe two channels ECG1 and ECG2. If it is determined that the deviceshould not transition to the not concerned state 302, i.e., both of thetwo heart rate estimates are greater than the heart rate threshold, theprocess is repeated using the signal generated during a nextthree-second window, Block 341.

According to an embodiment of the present invention, the heart ratethreshold level is set as 180 bpm, for example, and a single estimate ofheart rate (that occurs every three seconds) in at least one of the twochannels ECG1 and ECG2 that is less than the heart rate threshold levelwill suffice to cause the device to transition from the concerned state304 to the not concerned state 302, Yes in Block 372.

When the device transitions from the concerned state 304 to the armedstate 306, Yes in Block 370, processing continues to be triggered by athree-second time out as is utilized during the concerned state 304,described above. As illustrated in FIG. 7F, once the device transitionsfrom the concerned state 302 to the armed state 306, charging of thecapacitors is initiated, Block 600. During the charging of thecapacitors, the classification of segments for each channel ECG1 andECG2 as being either shockable or not shockable generated during theshock zone tests described above continues and once the next threeseconds of data has been acquired, Block 601, a determination is made aswhether the event continues to be a shockable event by determiningwhether a predetermined number of segments, such as the most recent twosegments for example, have been classified in both of the two channelsECG1 and ECG2 as not shockable, Block 602. If the predetermined numberof three second segments have been classified as not shockable,indicating that the event may possibly no longer be a shockable event,Yes in Block 602, the charging of the capacitors is stopped, Block 604,and a determination is made as to whether to transition to the notconcerned state 302, Block 606.

According to an embodiment of the present invention, the device willtransition from the armed state 306 to the not concerned state 302, Yesin Block 606, if certain termination requirements are met. For example,return to the not concerned state 302 occurs if, for both channels ECG1and ECG2 simultaneously, less than two out of the last threethree-second segments are classified as shockable, less than three outof the last eight three-second segments are classified as shockable, andthe most recent three second segment is classified as not shockable.Another possible criteria for returning to the not concerned state 302is the observation of 4 consecutive not shockable classifications inboth channel ECG1 and ECG2 simultaneously.

In addition to the two criteria described above, at least one of thecurrent heart rate estimates must be slower than the programmed ratethreshold 403, and capacitor charging must not in progress. If each ofthese requirements are satisfied, Yes in Block 606, the devicetransitions from the armed state 306 to the not concerned state 302.

If one or more of these requirements are determined not to be satisfied,return to the not concerned state is not indicated, No in Block 606, anda determination is then made as whether the shockable rhythm isredetected, Block 608, by determining whether predetermined redetectionrequirements have been satisfied. For example, a determination is madeas to whether a predetermined number of three-second segments in both ofthe two channels ECG1 and ECG2, such as two out of the most recent threefor example, have been classified as being shockable. If thepredetermined redetection requirements are not satisfied, No in Block608, the determination of whether to terminate delivery of the therapy,Block 606, is repeated so that the processing switches between thedetermination of whether to terminate delivery of therapy, Block 606 andthe determination as to whether the shockable event is redetected, Block608, until either the event has terminated and the device transitionsfrom the armed state 306 to the not concerned state 302 or the event isredetected. If the predetermined redetection requirements are met, Yesin Block 608, charging is re-initiated, Block 600, and the process isrepeated.

If, during the charging of the capacitors, the predetermined number ofthree second segments have not been classified as not shockable, No inBlock 602, a determination is made as to whether the charging of thecapacitors is completed, Block 610. As long as the predetermined numberof three second segments continue to be classified as shockable, No inBlock 602, charging of the capacitors continues until charging iscompleted. Once the charging of the capacitors is completed, Yes inBlock 610, a determination is made as to whether delivery of the therapyis still appropriate, Block 612, by determining whether predeterminedtherapy delivery confirmation requirements have been satisfied. Forexample, according to an embodiment of the present invention, thepredetermined therapy delivery confirmation requirements includedetermining whether, for both channels ECG1 and ECG2, at least five outof the last eight three-second segments are classified as beingshockable, and at least two of the last three three-second segments areclassified as being shockable. In addition, a determination is made asto whether the most recent three-second segment has been classified asbeing shockable for at least one of the two channels ECG1 and ECG2.

If the predetermined therapy delivery requirements have not beensatisfied, and therefore the delivery of the therapy is not confirmed,No in Block 612, the determination of whether to transition from thearmed state 306 to the not concerned state 302, Block 606, is repeated.If the predetermined therapy delivery requirements are satisfied, andtherefore the delivery of the therapy is confirmed, Yes in Block 612,the device transitions from the armed state 306 to the shock state 308.

As illustrated in FIG. 7G, once the device transitions from the armedstate 306 to the shock state 308, the therapy is delivered uponobservation of the first sensed R-wave, Block 630, the episode data isstored, Block 632, and the buffers for storing the eight three secondsegments are cleared, Block 634. Once a post shock timer, such as threeseconds for example, has expired, Yes in Block 636, the devicetransitions from the shock state 308 to Block 606 of the armed state306. Since, as described above, classification of at least threesubsequent three-second segments is required before the terminationdecision can be made in Block 606 subsequent to the delivery of therapyin the shock state 308, a determination based on the terminationrequirements cannot be initiated until at least twelve seconds after theinitial shock therapy was delivered. The termination and redetectionrequirements are then reviewed until one of the two requirements aresatisfied, i.e., the event is determined to have terminated, Yes inBlock 606, or the event is redetected, Yes in Block 608. If theredetection requirements are satisfied, the charging of the capacitorsis again initiated, Block 600, and processing in the armed state 306continues as described above until all available therapies have beenexhausted.

FIGS. 14A-14C are graphical representations illustrating the occurrenceof oversensing due to a slow monomorphic ventricular tachycardia with awide QRS complex. The graphical representation in FIG. 14A is anexemplary illustration of a slow (less than the VT/VF threshold, i.e.,180 bpm) monomorphic ventricular tachycardia with a wide QRS complex,commonly referred to as a slow VT, sensed via the first sensing channelECG1 and the second sensing channel ECG2. A resulting filtered andrectified signal 804 for the first sensing channel ECG1 is shown in FIG.14B and a resulting filtered and rectified signal 806 for the secondsensing channel ECG2 is shown in FIG. 14C. As can be seen in FIGS.14A-14C, oversensing tends to occur during slow VT because each QRScomplex results in two peak amplitudes 808 and 810 that exceed a sensingthreshold 812 in one or more of the sensing channels ECG1 and ECG2,resulting in double counting of R-waves in one or more of the sensingchannels ECG1 and ECG2. As a result, although the actual true heart rateoccurring during slow VT may be less than the VT/VF threshold (i.e., 180bpm), this oversensing resulting from the double counting of R-wavesduring such events causes the rhythm to be erroneously identified as aVT/VF rhythm.

FIG. 15 is a flowchart of a method for detecting cardiac events in amedical device according to an embodiment of the present invention. Themethod illustrated in FIG. 15 is similar to the method described abovein reference to FIG. 7A, but differs in that according to the method fordetecting cardiac events according to the embodiment of FIG. 15 includesthe additional process of detecting oversensing, which is describedbelow. The process leading up to the oversensing detection is the sameas described in FIG. 7A and will not be repeated here for brevity sake.

As illustrated in FIG. 15, once a VT/VF event has been determined to bepresent, i.e., the final heart rate estimates for both channels aredetermined to be greater than the predetermined VT/VF threshold, Yes inBlock 336, a determination is made as to whether this determinationoccurred as a result of oversensing, Block 820. If oversensing isdetermined to have occurred, Yes in Block 820, the determined heart rateis corrected for the oversensing, Block 822, and a determination is madeas to whether the corrected heart rate is greater than the predeterminedVT/VF threshold, Block 824. If the corrected heart rate is not greaterthan the predetermined VT/VF threshold, No in Block 824, the buffercontaining the 12 R-R intervals for the channel where the correctedheart rate not greater than the predetermined VT/VF threshold is updatedby removing the first R-sense, shifting the remaining eleven R-sensesamples back so that the second R-sense becomes the first R-sense, andso forth, and inserting the next detected R-sense, Block 322, as thetwelfth R-sense. A new current heart rate estimate is then determined,Block 323 and the process is repeated.

If oversensing is not determined to have occurred, No in Block 820, orif oversensing is determined to be present and the subsequentlygenerated corrected heart rate Block 822 is determined to be greaterthan the VT/VF threshold, Yes in Block 824, a VT/VF event is determinedto be present and the process transitions from the not concerned state302 to the concerned state 304.

FIGS. 16A and 16B are flowcharts of a method of determining whetheroversensing has occurred according to an embodiment of the presentinvention. According to an embodiment of the present invention, duringthe determination in Block 820 of whether oversensing has occurred, adetermination is made as to whether predetermined criteria associatedwith oversensing are met. For example, as illustrated in FIG. 16A, if,after determining that both heart rate estimates are greater than VT/VFthreshold, Block 336 of FIG. 15, both channels ECG1 and ECG2 weredetermined to be reliable, a spectral width is determined for eachchannel ECG1 and ECG2, Block 830, and a determination is made as towhich one of the spectral width for the first channel ECG1 and thespectral width of the second channel ECG2 is the maximum spectral width,Block 832. A determination is then made as to whether the maximumspectral width is less than or equal to a spectral width threshold,Block 834. According to an embodiment of the present invention, thespectral width threshold is set as −20, although it is understood thatany desired value may be utilized.

If the maximum spectral width is not less than or equal to the spectralwidth threshold, No in Block 834, oversensing is determined not to beoccurring, and the device transitions to the concerned state, Block 304.If the maximum spectral width is less than or equal to the spectralwidth threshold, Yes in Block 834, a metric of signal energy content isdetermined for each channel ECG1 and ECG2, Block 836, and adetermination is made as to which one of the metric of signal energycontent for the first channel ECG1 and the metric of signal energycontent for the second channel ECG2 is the minimum metric of signalenergy content, Block 838. A determination is then made as to whetherthe minimum metric of signal energy content is greater than a metric ofsignal energy content threshold, Block 840. According to an embodimentof the present invention, the metric of signal energy content is anormalized mean rectified amplitude, generated as described above, andthe metric of signal energy content threshold is 40.

If the minimum metric of signal energy content is not greater than themetric of signal energy content threshold, No in Block 840, oversensingis determined to not be occurring, and the device transitions from thenot concerned state 302 to the concerned state 304. If the minimummetric of signal energy content is greater than the metric of signalenergy content threshold, Yes in Block 840, a heart rate metricdifference is determined for each channel ECG1 and ECG2, Block 842, anda determination as to which one of the heart rate metric difference forthe first channel ECG1 and the heart rate metric difference for thesecond channel ECG2 is the maximum heart rate metric difference, Block844. A determination is then made as to whether the maximum heart ratemetric difference is less than or equal to a heart rate metricdifference threshold, Block 846, such as 52.5 ms for example. If themaximum heart rate metric difference is greater than the heart ratemetric difference threshold, No in Block 846, oversensing is determinedto not be occurring, and the device transitions from the not concernedstate 302 to the concerned state 304. If the maximum heart rate metricdifference is less than or equal to the heart rate metric differencethreshold, Yes in Block 846, oversensing is determined to likely beoccurring and the corrected rate is determined, Block 822 of FIG. 15,described below.

According to an embodiment of the present invention, the heart ratemetric difference is derived from the 12 RR intervals currently storedin the buffer so that the heart rate metric difference for each channelECG 1 and ECG2 is generated by first determining the trimmed mean of thethird through the tenth RR intervals TM=Mean {RR₃: RR₁₀}, andcalculating an absolute difference between the second RR interval andthe trimmed mean |RR₂−TM|, between the fifth RR interval and the trimmedmean |RR₅−TM|, between the eight RR interval and the trimmed mean|RR₈−TM|, and between the eleventh RR interval and the trimmed mean|RR₁₁−TM|. The heart rate metric difference for the channel is then setequal to the average of the four calculated absolute differences,

As illustrated in FIG. 16B, if, after determining that both heart rateestimates are greater than the VT/VF threshold, Block 336, only one ofthe channels ECG1 and ECG2 was determined to be reliable, No in Block328 and Yes in Block 330, the oversensing criteria are applied usingonly the reliable channel. For example, if the first channel ECG1 wasdetermined to be unreliable and the second channel ECG2 was determinedto be the reliable channel, a spectral width is determined for thereliable channel ECG2, Block 850, and a determination is made as towhether the spectral width is less than or equal to the spectral widththreshold, Block 852.

If the spectral width is not less than or equal to the spectral widththreshold, No in Block 852, oversensing is determined not to beoccurring, and the device transitions to the concerned state, Block 304.If the spectral width is less than or equal to the spectral widththreshold, Yes in Block 852, a metric of signal energy content isdetermined for the reliable channel ECG2, Block 854, and a determinationis made as to whether the metric of signal energy content is greaterthan the metric of signal energy content threshold, Block 856.

If the metric of signal energy content is not greater than the metric ofsignal energy content threshold, No in Block 856, oversensing isdetermined to not be occurring, and the device transitions from the notconcerned state 302 to the concerned state 304. If the metric of signalenergy content is greater than the metric of signal energy contentthreshold, Yes in Block 856, a heart rate metric difference isdetermined for the reliable channel ECG2, Block 858, and a determinationas to whether the heart rate metric difference is less than or equal tothe heart rate metric difference threshold, Block 860. If the heart ratemetric difference is greater than the heart rate metric differencethreshold, No in Block 860, oversensing is determined to not beoccurring, and the device transitions from the not concerned state 302to the concerned state 304. If the heart rate metric difference is lessthan or equal to the heart rate metric difference threshold, Yes inBlock 860, oversensing is determined to likely be occurring and thecorrected rate is determined, Block 822 of FIG. 15, using the ratecorrection technique described below.

It is understood that while the method of determining whetheroversensing has occurred described above in reference to FIGS. 16A and16B includes three oversensing characteristics that must be satisfied,spectral width, the metric of signal energy content, and the heart ratemetric difference, in order for oversensing to be detected, the presentinvention may require that a combination of any two, or only one of thethree oversensing characteristics to be satisfied in order foroversensing to be detected. In addition, while the three oversensingcharacteristics are described in terms of the spectral width being thefirst to be determined, followed by the metric of signal energy content,and then the heart rate metric difference, if more than one of theoversensing characteristics is utilized, they may occur in any order.

FIG. 17 is a flowchart of a method of determining whether oversensinghas occurred according to an embodiment of the present invention. Asillustrated in FIGS. 7B and 17, once the device has transitioned fromthe not concerned state 302 to the concerned state 304, and it isdetermined that noise was detected in only one of the channels, Yes inBlock 346, a determination is made as to whether oversensing hasoccurred in the clean channel, Block 862. Since the oversensing isdetermined for only one of the two channels ECG1 and ECG 2, i.e., thenon-noisy or clean channel, the oversensing criteria as describe abovein reference to FIG. 16B is utilized in the oversensing determination ofBlock 862. If oversensing is not detected, No in Block 862, theabove-described determination of whether there are regular intervals inthe clean channel, Block 348, is made and the process continues in FIG.7B as described above.

If oversensing is determined to be detected, a corrected rate for theclean channel is determined, Block 863, using the rate correctionprocess described below, and a determination is then made as to whetherthe corrected rate is greater than the VT/VF detection threshold, Block864. If the corrected heart rate is greater than the VT/VF threshold,the above-described determination of whether there are regular intervalsin the clean channel, Block 348, is made and the process continues inFIG. 7B as described above. If the corrected rate is not greater thanthe VT/VF threshold, No in Block 864, the clean channel is classified asbeing unshockable, Block 865, and the determination is made as towhether the device should transition from the concerned state 304 to thearmed state 306, Block 370, as described above.

If it is determined that both channels ECG1 and ECG2 are noise-free,i.e., both are clean channels, No in Block 346, a determination is madeas to whether oversensing has occurred, Block 866, using both channelsECG1 and ECG2. Since the oversensing is determined using both channelsECG1 and ECG 2, the oversensing criteria as describe above in referenceto FIG. 16A is utilized in the oversensing determination of Block 866.If oversensing is not detected, No in Block 866, the determination ofwhether there are regular intervals in both channels ECG1 and ECG2,Block 356, is made and the process continues in FIG. 7B as describedabove.

If oversensing is determined to be detected, a corrected rate isdetermined for each channel ECG1 and ECG2, Block 867, using the ratecorrection process described below, and a determination is then made asto whether the corrected rates for both channels ECG 1 and ECG2 aregreater than the VT/VF detection threshold, Block 868. If the correctedheart rates for both channels ECG1 and ECG2 are greater than the VT/VFthreshold, Yes in Block 868, the determination of whether there areregular intervals in both channels, Block 356, is made and the processcontinues in FIG. 7B as described above. If the corrected rates for bothchannels ECG1 and ECG2 are not greater than the VT/VF threshold, No inBlock 868, a determination is made as to whether the corrected rate forboth of the channels ECG1 and ECG2 is less than or equal to the VF/VFthreshold, Block 869. If the corrected rates for both of the channelsECG1 and ECG2 is less than or equal to the VF/VF threshold, Yes in Block869, i.e., both channels ECG1 and ECG2 are slow, and therefore bothchannels ECG1 and ECG 2 are classified as being not shockable, Block870, and the determination is made as to whether the device shouldtransition from the concerned state 304 to the armed state 306, Block370, as described above.

If the corrected rates for both of the channels ECG1 and ECG2 are notless than or equal to the VF/VF threshold, No in Block 869, indicatingthat the corrected rate for one of the channels ECG1 and ECG2 is lessthan or equal to the VF/VF threshold and the corrected rate for theother of the channels ECG1 and ECG2 is greater than the VT/VF threshold,the channel that was determined to be less than the VT/VF threshold,i.e., the slow channel, is classified as being non-shockable, Block 871,and a determination is made as to whether there are regular intervals inthe channel that was greater than the VT/VF threshold, i.e., the fastchannel, Block 872, using the regular intervals processing as describedabove, indicating the channel can be classified as being stable.

If the fast channel is determined to have regular intervals, adetermination is made as to whether the fast channel is in the VT shockzone, Block 873, using the VT shock zone determination described above.If the fast channel is in the VT shock zone, the fast channel isclassified as shockable, Block 874, and the determination is made as towhether the device should transition from the concerned state 304 to thearmed state 306, Block 370, as described above. If the fast channel isnot in the VT zone, the fast channel is classified as being notshockable, Block 875, and the determination is made as to whether thedevice should transition from the concerned state 304 to the armed state306, Block 370, as described above.

If the fast channel is determined not to have regular intervals, No inBlock 872, a determination is made as to whether the fast channel is inthe VF shock zone, Block 876, using the VF shock zone criteria describedabove. If the fast channel is in the VF shock zone, the fast channel isclassified as shockable, Block 874, and the determination is made as towhether the device should transition from the concerned state 304 to thearmed state 306, Block 370, as described above. If the fast channel isnot in the VF zone, the fast channel is classified as being notshockable, Block 875, and the determination is made as to whether thedevice should transition from the concerned state 304 to the armed state306, Block 370, as described above.

FIG. 18 is a flowchart of a method of determining whether oversensinghas occurred according to an embodiment of the present invention. Asillustrated in FIG. 18, in order to determine whether the fast channelhas regular intervals, Block 872 of FIG. 17, and can therefore beclassified as being relatively stable, predetermined maximum and minimumintervals for the fast channel are identified, Block 877, using theupdated buffer of 12 RR-intervals. According to one embodiment of thepresent invention, the largest RR-interval and the sixth largestRR-interval of the twelve RR-intervals are utilized as the maximuminterval and the minimum interval, respectively.

The difference between the maximum RR-interval and the minimumRR-interval of the 12 RR-intervals is calculated, Block 878, to generatean interval difference associated with the fast channel. A determinationis made as to whether the interval difference is greater than apredetermined stability threshold, Block 879, such as 110 milliseconds,for example.

If the interval difference is greater than the stability threshold, thesegment is classified as an unstable segment, Block 880, and adetermination is made as to whether the signal associated with the fastchannel is within a predetermined VF shock zone, Block 876, describedabove. If the interval difference is less than or equal to the stabilitythreshold, No in Block 879, the device determines whether the minimumRR-interval associated with the fast channel is greater than a minimuminterval threshold, Block 881, such as 200 milliseconds, for example.

If the minimum RR interval is less than or equal to the minimum intervalthreshold, the segment is classified as an unstable segment, Block 880,and a determination is made as to whether the signal associated with thefast channel is within a predetermined VF shock zone, Block 876,described above. If the minimum interval is greater than the minimuminterval threshold, the device determines whether the maximumRR-interval associated with the fast channel is less than or equal to amaximum interval threshold, Block 882, such as 333 milliseconds forexample. If the maximum interval is greater than the maximum intervalthreshold, the segment is classified as an unstable segment, Block 880,and a determination is made as to whether the signal associated with thefast channel is within a predetermined VF shock zone, Block 876,described above.

If the maximum interval is less than or equal to the maximum intervalthreshold, the segment is classified as a stable segment, Block 883, anda determination is made as to whether the signal associated with thefast channel is within a predetermined VT shock zone, Block 873,described above.

It should be noted that, as described above in reference to FIG. 7F,when the device transitions from the concerned state 304 to the armedstate 306, Yes in Block 370, processing continues to be triggered by athree-second time out as is utilized during the concerned state 304,described above, and once the device transitions from the concernedstate 302 to the armed state 306, charging of the capacitors isinitiated, Block 600. During the charging of the capacitors, theclassification of segments for each channel ECG1 and ECG2 as beingeither shockable or not shockable generated during the shock zone testsdescribed above continues, and, according to an embodiment of thepresent invention, this classification of the segments while in thearmed state 306 includes the determination of whether oversensing hasoccurred, along with the generation of the corrected rate whenoversensing has occurred, as described above.

FIGS. 19A and 19B are graphical representations of determining acorrected heart rate in response to oversensing according to anembodiment of the present invention. FIGS. 20A and 20B are flowcharts ofa method of determining a corrected heart rate in response tooversensing according to an embodiment of the present invention. Asillustrated in FIGS. 19A, 19B and FIGS. 20A and 20B, during thedetermination of an updated or corrected heart rate in response todetermining the presence of oversensing, the device first determines amaximum amplitude 886 and an absolute value of the minimum amplitude forthe current three second window, Block 900, and determines whether themaximum amplitude for the three second window is greater than theabsolute value of the minimum amplitude for the three second window,Block 901. If the maximum amplitude for the three second window is notgreater than the absolute value of the minimum amplitude for the threesecond window, No in Block 901, the device uses the minimum amplitudesto determine the corrected rates, Block A, described below in FIG. 20B.

In order to determine an updated or corrected heart rate, adetermination is made as to whether certain predetermined criteriaassociated with the corrected heart rate are met. For example, if themaximum amplitude 886 for the three second window is greater than theabsolute value of the minimum amplitude for the three second window, Yesin Block 901, the device divides the three second window 884 into apredetermined number of window sub-segments 885, Block 902. A localmaximum amplitude 887 is determined for one of the sub-segments 885,Block 904, and a determination is made as to whether the local maximum887 for that sub-segment 885 is greater than a predetermined localmaximum amplitude threshold, Block 906. According to an embodiment ofthe present invention, the predetermined local maximum amplitudethreshold is a percentage of the overall maximum amplitude 886 for thesensing window, such as 65% of the overall maximum amplitude 886.

If the current local maximum amplitude 887 is not greater than the localmaximum amplitude threshold, No in Block 906, no determination of amaximum amplitude is made for that sub-segment, Block 908, and thedevice determines whether the local maximum amplitude 887 and localamplitudes 888 have been determined for all of the four sub-segments885, Block 922, described below. If the current local maximum amplitude887 is greater than the local maximum amplitude threshold, Yes in Block906, the device determines whether the sub-segment 885 includes a localamplitude 888 that is greater than a local amplitude threshold, Block918.

According to an embodiment of the present invention, the local amplitudethreshold of Block 918 is a percentage of the local maximum amplitude887 for that sub-segment 885, such as 70% of the local maximum amplitude887 for that sub-segment 885, for example. If the sub-segment 885 doesnot include a local amplitude 888 that is greater than the localamplitude threshold, No in Block 918, a determination cannot be made forthat sub-segment 885, Block 920, and the device determines whether alocal maximum amplitude 887 and local amplitudes 888 have beendetermined for all of the four sub-segments 885, Block 922, describedbelow.

If the sub-segment 885 includes at least one local amplitude 888 that isgreater than the local amplitude threshold, Yes in Block 918, or if nodetermination of a local maximum amplitude was made, Block 908, thedevice determines whether the local maximum amplitude 887 and localamplitudes 888 have been determined for all of the four sub-segments885, Block 922. If all of the local maximum amplitude 887 and localamplitudes 888 have not been determined for all of the sub-segments 885,Yes in Block 922, the process is repeated for the next sub-segment 885.

In order to obtain a corrected heart rate for a three second window, thedevice must be successful in locating a local maximum amplitude 887 andat least one local amplitude 888 in a predetermined number of thesub-segments, such as two sub-segments for example. Therefore, once thedetermination of a local maximum amplitude 887 and the local amplitudes888, Blocks 904-918, has been made for all of the sub-segments 885, Noin Block 922, a determination is made as to whether a local maximumamplitude 887 and at least one local amplitude 888 were found to existin the predetermined number of sub-segments 885, Block 924. If a localmaximum amplitude 887 and at least one local amplitude 888 greater thanthe local amplitude threshold were not found for the predeterminednumber of sub-segments 885, No in Block 924, no corrected heart rate isdetermined for the three second window, Block 914. If a local maximumamplitude 887 and at least one local amplitude 888 greater than thelocal amplitude threshold was found for the predetermined number ofsub-segments 885, Yes in Block 924, rates are determined for each of thefour sub-segments 885, Block 926, and the corrected rate is determinedusing the determined sub-segment rates, Block 928.

If the maximum amplitude for the three second window is not greater thanthe absolute value of the minimum amplitude for the three second window,No in Block 901, the device uses the minimum amplitudes to determine thecorrected rates, Block A in FIG. 20A. In particular, as illustrated inFIG. 20B, the device divides the three second window 934 into apredetermined number of window sub-segments 935, Block 940, anddetermines an overall minimum amplitude 936 associated with the entirewindow, Block 942. A local minimum amplitude 937 is determined for oneof the sub-segments 935, Block 944, and a determination is made as towhether the local minimum 937 for that sub-segment 935 is less than apredetermined local minimum amplitude threshold, Block 946. According toan embodiment of the present invention, the predetermined local minimumamplitude threshold is a percentage of the overall minimum amplitude 936for the sensing window, such as 65% of the overall minimum amplitude936.

If the current local minimum amplitude 937 is not less than the localminimum amplitude threshold, No in Block 946, no determination of aminimum amplitude is made for that sub-segment, Block 948, and thedevice determines whether the local minimum amplitude 937 and localamplitudes 938 have been determined for all of the four sub-segments935, Block 956, described below. If the current local minimum amplitude937 is less than the local minimum threshold, Yes in Block 946, thedevice determines whether the sub-segment 935 includes local amplitudes938 that are less than a local amplitude threshold, Block 952.

According to an embodiment of the present invention, the local amplitudethreshold of Block 952 is a percentage of the local minimum amplitude937 for that sub-segment 935, such as 70% of the local minimum amplitude937 for that sub-segment 935, for example. If the sub-segment 935 doesnot include at least one local amplitude 938 that is less than the localamplitude threshold, No in Block 952, a determination cannot be made forthat sub-segment 935, Block 954, and the device determines whether thelocal minimum amplitude 937 and local amplitudes 938 have beendetermined for all of the four sub-segments 935, Block 956, describedbelow.

If the sub-segment 935 includes at least one local amplitude 938 that isless than the local amplitude threshold, Yes in Block 952, or if nodetermination of a local minimum amplitude was made, Block 948, thedevice determines whether the local minimum amplitude 937 and localamplitudes 938 have been determined for all of the four sub-segments935, Block 956. If all of the local minimum amplitudes 937 and localamplitudes 938 have not been determined for all of the sub-segments 935,Yes in Block 956, the process is repeated for the next sub-segment 935.

In order to obtain a corrected heart rate for a three second window, thedevice must be successful in locating a local minimum amplitude 937 andat least one local amplitude 938 in a predetermined number of thesub-segments, such as two sub-segments for example. Therefore, once thedetermination of a local minimum amplitude 937 and the local amplitudes938, Blocks 942-952, has been made for all of the sub-segments 935, Noin Block 956, a determination is made as to whether a local minimumamplitude 937 and at least one local amplitude 938 less than the localamplitude threshold has been found for the predetermined number ofsub-segments 935, Block 958. If a local minimum amplitude 937 and atleast one local amplitude 938 less than the local amplitude thresholdwere not found for the predetermined number of sub-segments 935, No inBlock 958, no corrected heart rate is determined for the three secondwindow, Block 960. If a local minimum amplitude 937 and at least onelocal amplitude 938 less than the local amplitude threshold were foundfor the predetermined number of sub-segments 935, Yes in Block 958,rates are determined for each of the four sub-segments 935, Block 962,and the corrected rate is determined using the determined sub-segmentrates, Block 964.

FIG. 21 is a flowchart of a method for determining a corrected rateaccording to an embodiment of the present invention. The flowchart ofFIG. 21 describes the determination of the sub-segment rates when themaximum amplitude is utilized, Yes in Block 901 of FIG. 20A. Thedetermination of the sub-segment rates when the minimum amplitudes areutilized, No in Block 901, is similar to the determination when themaximum amplitude is utilized, differing only in that minimum amplitudesare used in place of the maximum amplitudes, and therefore is notincluded merely for brevity sake.

In particular, according to an embodiment of the present invention, inorder to determine sub-segment rates, Block 926, the device determinesinterval rates associated with the local maximum amplitude 887 and thelocal amplitudes 888 within a sub-segment 885 that were determined inBlock 918, and a sub-segment rate for each of the sub-segments 885 isdetermined based on the RR interval rates, Block 932. For example, asillustrated in FIG. 19A, one of the sub-segments 885 includes the localmaximum amplitude 887 and two local amplitudes 888 so that two RRintervals, RR1 and RR2 are determined as the intervals for thatsub-segment 885 in Block 930. The rate for that sub-segment 885 isdetermined in Block 932 based on RR intervals RR1 and RR2, such as basedon the average of the RR intervals RR1 and RR2. If a sub-segment ratehas not been determined for all of the sub-segments 885 for which alocal maximum amplitude 887 and at least one local amplitude 888 werepreviously determined, Yes in Block 334, the device determines asub-segment rate for the next sub-segment 885, Blocks 930 and 932.

For example, the next sub-segment 885 includes the local maximumamplitude 887 and one local amplitude 888, so that the sub-segment rateis equal to the RR interval RR3. The next sub-segment 885 includes thelocal maximum amplitude 887 and one local amplitude 888, so that thesub-segment rate is equal to the RR interval, RR4. No sub-segment rateis determined for the last sub-segment 885 since, although the lastsub-segment included a local maximum amplitude, which corresponded tothe overall maximum amplitude for the window 886, the last sub-segmentdid not include at least one additional local amplitude 888 that wasdetermined in Block 918 to be greater than the local amplitudethreshold.

Once a rate has been determined for each of the qualified sub-segments885, No in Block 934, the device determines the corrected rate for thethree second window, Block 936, based on the determined sub-segmentrates. For example, the device sorts the determined sub-segment ratesfrom fastest to slowest, and uses the average of a predetermined numberof the sorted sub-segment rates to determine the corrected rate. Inparticular, according to an embodiment of the present invention, thedevice makes the determination based on the number of sub-segment ratesthat were determined, so that if a sub-segment rate was determined forall four of the sub-segments, the corrected rate is determined as theaverage of the second and third sub-segment rates of the sortedsub-segment rates. If a sub-segment rate was determined for three of thefour sub-segments, the corrected rate is determined as the average ofthe second and third sub-segment rates of the sorted sub-segment rates,and if a sub-segment rate was determined for only two of thesub-segments, the corrected rate is determined as the average of thefirst and second sub-segment rates of the sorted sub-segment rates.

FIG. 22 is a flowchart of a method for determining a corrected rateaccording to an embodiment of the present invention. FIG. 23 is anexemplary schematic diagram of a buffer of RR intervals generatedaccording to an embodiment of the present invention. In particular, FIG.23 illustrates an example of a buffer of twelve RR intervals 996 thatwere stored during an instance of oversensing due to a slow monomorphicventricular tachycardia with a wide QRS complex, in which the firstinterval is a long interval L, the second interval is a short intervalS, the third interval is a long interval L, and so forth.

According to another embodiment of the present invention, in order todetermine the corrected heart rate in response to the determination ofoversensing, the device identifies short-long interval patterns bylooking sequentially at each RR interval, Block 970, starting with thefirst RR interval of the stored buffer of 12 RR intervals, andgenerating an update RR interval to be stored in an updated RR intervalbuffer based on a determination as to whether the interval is a shortinterval or a long interval. In particular, as illustrated in FIG. 22,in order to identify short-long interval patterns, the device obtains anRR interval, Block 970, and determines whether the RR interval is lessthan or equal to an interval threshold, Block 972. If the current RRinterval is not less than the interval threshold, No in Block 972, theRR interval is likely a long interval and therefore not a first intervalof a short-long interval pattern. Therefore the current RR interval isset as the next RR interval of the updated RR intervals, Block 974. Ifthe current RR interval is less than or equal to the interval threshold,Yes in Block 972, the current RR interval is likely a short interval,and therefore the device obtains the next RR interval immediatelysubsequent to the current RR interval, Block 976, and sets the nextupdated RR interval equal to the sum of the current RR interval, i.e.,the short RR interval and the next RR interval, Block 978.

According to an embodiment of the present invention, the intervalthreshold is the sum of the blanking period, i.e., 180 ms for example,and a rate correction constant associated with a maximum width of theparticular waveform that the device is attempting to identify. Forexample, in an embodiment of the present invention, the rate correctionconstant is set as 20 ms when the device is attempting to identifyinstances of oversensing due to a slow monomorphic ventriculartachycardia with a wide QRS complex.

Once an updated RR interval has been determined based on a single longRR interval, Block 974, or based on the sum of a short interval and anext interval, Block 978, the device then determines in Block 980whether each one of the 12 buffered RR intervals have been eitheridentified as being a long interval, Block 972, and therefore used aloneto generate a next update interval, or utilized in combination with apreceding interval determined to be a short interval to generate thenext updated interval Blocks 976-978. If all of the 12 buffered RRintervals have not been utilized to generate a next updated RR interval,Yes in Block 980, the process is repeated with the next availablebuffered RR interval, Block 970. If all of the 12 buffered RR intervalshave been utilized to generate an updated RR interval, No in Block 980,the device sets the corrected rate based on the buffer of updated RRintervals, Block 982. For example, according to an embodiment of thepresent invention, the device sets the corrected rate equal to the meanof the second through the last updated RR interval of the buffer ofupdated RR intervals.

As illustrated in FIGS. 22 and 23, since the first interval of buffer996 is a long interval, No in Block 972, the first updated interval of abuffer of updated RR intervals 998 is set equal to the first interval ofbuffer 996. Since the second interval of buffer 996 is a short interval,the second updated interval of buffer 998 is set equal to the sum of thesecond and third intervals of buffer 996. The process is repeated,resulting in the buffer of updated RR intervals 998 containing sevenupdated intervals, so that the corrected rate is determined in Block 982as the mean of the second through the seventh updated RR interval frombuffer 998.

According to an embodiment of the present invention, the intervalthreshold in Block 972 may correspond to a trimmed mean of the buffered12 RR intervals, such as a trimmed mean of the third through the tenthsorted RR intervals, as utilized above for example TM. In this way, inthe determination of the corrected rate, the device first sorts the 12buffered RR intervals from smallest to largest, and sets the intervalthreshold utilized in Block 972 as the trimmed mean of the third throughtenth RR interval. Once the interval threshold has been set, the ratecorrection process, Blocks 970-982, is performed as described above. Asa result, the corrected rate is determined in response to both a sortedand a non-sorted buffer of the buffer of 12 RR intervals.

FIG. 24 is a flowchart of a method for determining a corrected rateaccording to an embodiment of the present invention. FIG. 25 is anexemplary schematic diagram of a buffer of RR intervals generatedaccording to an embodiment of the present invention. In particular, FIG.25 illustrates an example of a buffer of twelve RR intervals 997 thatwere stored during an instance of oversensing due to a slow monomorphicventricular tachycardia with a wide QRS complex, in which the firstinterval is a long interval L, the second interval is a short intervalS, the third interval is a long interval L, the fourth and fifthintervals are short intervals, and so forth. In order to further insurethat rate correction is only applied when a short-long RR intervalpattern exists, the device performs the rate correction according toanother embodiment of the present by looking sequentially at twoadjacent RR intervals, Block 984, starting with the first two RRintervals of the stored buffer of 12 RR intervals, generates an updatedRR interval buffer based on whether the first RR interval of theadjacent RR intervals is a short interval and the second RR interval isa long interval, and determines the corrected rate based on the updatedintervals in the updated RR interval buffer. In particular, asillustrated in FIG. 24, the device obtains adjacent RR intervals, Block984, starting with the first and second RR interval for example, anddetermines whether the first RR interval of the two adjacent RRintervals is less than or equal to an interval threshold, Block 986. Ifthe first RR interval is not less than or equal to the intervalthreshold, No in Block 986, the first RR interval is likely a longinterval, negating the possibility that the current two intervalscorrespond to a short-long interval sequence, and therefore the first RRinterval is set as the next updated RR interval of the buffer of updatedRR intervals, Block 988.

If the first RR interval is less than or equal to the intervalthreshold, Yes in Block 986, the current RR interval is likely a shortinterval, and therefore the device determines whether the second RRinterval, i.e., the subsequent interval adjacent to the first RRinterval, is greater than the interval threshold, Block 990. If thesecond RR interval is not greater than the interval threshold, No inBlock 990, both the first RR interval and the second RR interval arelikely short intervals, negating the possibility that the current two RRintervals correspond to a short-long interval sequence, and thereforethe first RR interval is set as the next updated RR interval of thebuffer of updated RR intervals, Block 988. If the first RR interval isless than or equal to the interval threshold, Yes in Block 986, and thesecond RR interval is greater than the interval threshold, Yes in Block990, indicating that the first RR interval is a short interval and thesecond RR interval is a long interval, the device sets the next updatedRR interval in the stored buffer of RR intervals based on the twocurrent intervals, Block 992. For example, according an embodiment ofthe present invention, the device sets the next updated RR intervalequal to the sum of the current first RR interval and second RRinterval.

According to an embodiment of the present invention, the intervalthreshold of Blocks 986 and 990 is the sum of the blanking period, i.e.,180 ms for example, and a rate correction constant associated with amaximum width of the particular waveform that the device is attemptingto identify. For example, in an embodiment of the present invention, therate correction constant is set as 60 ms when the device is attemptingto identify instances of oversensing due to a slow monomorphicventricular tachycardia with a wide QRS complex.

Once an updated RR interval has been determined based on a single longRR interval, Block 988, or based on the sum of a short interval and along interval, Block 992, the device then determines in Block 994whether each one of the 12 buffered RR intervals have been eitheridentified as being a long interval, Block 986, and therefore used aloneto generate a next update interval, Block 988, or utilized incombination with an adjacent RR interval determined to form a short-longinterval sequence, Block 992. If all of the 12 buffered RR intervalshave not been utilized to generate a next updated RR interval, Yes inBlock 994, the process is repeated with the next available adjacentbuffered RR intervals, Block 984. If all of the 12 buffered RR intervalshave been utilized to generate a next updated RR interval, No in Block994, the device sets the corrected rate based on the updated RRintervals, Block 996. For example, according to an embodiment of thepresent invention, the device sets the corrected rate equal to the meanof the second through the last updated RR intervals of the updated RRintervals.

As illustrated in FIGS. 24 and 25, since the first interval of buffer997 is a long interval, No in Block 986, the first updated interval of abuffer of updated RR intervals 999 is set equal to the first interval ofbuffer 997. Since the second interval of buffer 997 is a short interval,Yes in Block 986, and the third interval is a long interval, Yes inBlock 990, the second updated interval of buffer 999 is set equal to thesum of the second and third RR intervals of buffer 997. Since the fourthRR interval of buffer 997 is a short interval, Yes in Block 986, and thefifth RR interval is a short interval, No in Block 990, the thirdupdated interval of buffer 999 is set equal to the fourth interval ofbuffer 997. Since the fifth interval was not included in generating thethird updated interval, the device utilizes the fifth and sixthintervals of buffer 997 in the next iteration, resulting in adetermination that the fifth RR interval of buffer 997 is a shortinterval, Yes in Block 986, and the sixth RR interval is a longinterval, Yes in Block 990, so that the fourth updated RR interval ofbuffer 999 is set equal to the sum of the fifth and sixth intervals ofbuffer 997. The process is repeated, resulting in the buffer of updatedRR intervals 998 containing eight updated intervals, so that thecorrected rate is determined in Block 996 as the mean of the secondthrough the seventh updated RR interval from buffer 999.

The present invention may utilize other embodiments for determining thecorrected rate in place of the embodiments described above. For example,according to another embodiment of the present invention, in order todetermine the corrected heart rate in response to oversensing, thedevice sorts the 12 buffered RR intervals from smallest to largest sothat the corrected rate is determined based on predetermined RRintervals from the sorted RR intervals. For example, according to anembodiment of the present invention, the corrected rate is determined asthe sum of the mean of second through fifth smallest RR interval and themean of the eighth through eleventh RR interval.

According to another embodiment of the present invention, in order todetermine the corrected heart rate in response to oversensing, thedevice sorts the 12 buffered RR intervals from smallest to largest sothat the corrected rate is determined based on one or more predeterminedRR intervals from the sorted RR intervals and the spectral width of thesignal for the three-second segment associated with each channel ECG1and ECG2, determined as described above. For example, according to anembodiment of the present invention, the corrected rate is determined asthe sum of the 9^(th) fastest RR interval of the sorted RR intervals andthe spectral width associated with the three second segment for thechannel ECG1 or ECG2.

FIG. 26 is a flowchart of a method for detecting cardiac events in amedical device according to an embodiment of the present invention. Themethod described in FIG. 26 is similar to the method described above inreference to FIG. 15, and includes the determination of whetheroversensing has occurred in a situation where only one of the sensingchannels ECG1 and ECG2 has been determined to be reliable, describedabove in reference to FIG. 16B. Therefore, a description of those stepsalready described above in reference to FIGS. 15 and 16B will not berepeated for brevity sake. The embodiment of FIG. 26 differs only inthat it includes additional steps associated with a timer forcontrolling the rate at which the oversensing determination isperformed, which results in a reduction in the amount of processing thatoccurs during the oversensing determination, thereby increasing thebattery life of the device.

In particular, as illustrated in FIG. 26, once the device has determinedthe final heart rate estimate for each channel ECG1 and ECG2 using themethods described above in reference to Blocks 322-334 of FIG. 7A, forexample, a determination is made as whether a timer has expired, Block861. The timer of Block 861, which is initialized as being expired,i.e., equal to zero, is set during the oversensing determination as willbe described below. According to an embodiment of the present invention,the timer is set as 750 ms, which corresponds to the time associatedwith a single sub-segment of the three-second window, although anydesired time period for controlling the rate at which the oversensingdetermination is performed may be utilized.

If the timer has not expired, i.e., at least one of the oversensingcriteria has been satisfied within the last 750 ms, No in Block 861, thedetermination of the heart rate estimate for each channel is repeated,Blocks 322-334. If the timer has expired, i.e., at least one of theoversensing criteria has not been satisfied within the last 750 ms, Yesin Block 861, the determination is then made as to whether the finalheart rate estimates is greater than the predetermined VT/VF threshold,Block 336, described above. If the final heart rate estimates are notgreater than the VT/VF threshold, a new heart rate estimate isdetermined, Blocks 322-334. If the final heart rate estimates aregreater than the VT/VF threshold, the oversensing determination is madefor the current generated heart rate estimate. According to theembodiment of the present invention illustrated in FIG. 26, theoversensing determination includes all three of the previously describedoversensing characteristics, spectral width, the metric of signal energycontent, and the heart rate metric difference, with the heart ratemetric difference being the initial oversensing characteristic. However,as mentioned above, embodiments of the present invention may include anynumber or combination of the oversensing characteristics may beutilized, performed in any desired sequence.

In particular, for example, if the first channel ECG1 was determined tobe unreliable and the second channel ECG2 was determined to be thereliable channel, a heart rate metric difference is determined for thereliable channel ECG2, Block 858, and a determination as to whether theheart rate metric difference is less than or equal to the heart ratemetric difference threshold, Block 860. If the heart rate metricdifference is greater than the heart rate metric difference threshold,No in Block 860, oversensing is determined to not be occurring, and thedevice transitions from the not concerned state 302 to the concernedstate 304. If the heart rate metric difference is less than or equal tothe heart rate metric difference threshold, Yes in Block 860, the timeris set, Block 867. A spectral width is then determined for the reliablechannel ECG2, Block 850, and a determination is made as to whether thespectral width is less than or equal to the spectral width threshold,Block 852.

If the spectral width is not less than or equal to the spectral widththreshold, No in Block 852, oversensing is determined not to beoccurring, and the device transitions to the concerned state, Block 304.If the spectral width is less than or equal to the spectral widththreshold, Yes in Block 852, a metric of signal energy content isdetermined for the reliable channel ECG2, Block 854, and a determinationis made as to whether the metric of signal energy content is greaterthan the metric of signal energy content threshold, Block 856.

If the metric of signal energy content is not greater than the metric ofsignal energy content threshold, No in Block 856, oversensing isdetermined to not be occurring, and the device transitions from the notconcerned state 302 to the concerned state 304. If the metric of signalenergy content is greater than the metric of signal energy contentthreshold, Yes in Block 856, oversensing is determined to likely beoccurring and the corrected rate is determined, Block 822 of FIG. 15,using the rate correction technique described above. If the correctedrate is greater than the predetermined VT/VF threshold, Yes in Block824, oversensing is determined to not be occurring, and the devicetransitions from the not concerned state 302 to the concerned state 304.

If the corrected heart rate is not greater than the predetermined VT/VFthreshold, No in Block 824, a new current heart rate estimate is thendetermined, Blocks 322-332 and the process is repeated. However, sinceno oversensing was determined to be occurring for the previous heartrate estimate, and therefore the timer was initiated in Block 867, oncethe next heart rate estimate has been determined, Block 334, thedetermination of oversensing will be delayed until a desired amount ofnew sensing data has been analyzed, since the timer will not be expired,No in Block 861. While the amount of time necessary for the oversensingdetermination and the rate correction to occur will vary depending onthe number of oversensing characteristics utilized, the inventor hasfound that the oversensing determination process of FIG. 26 may takeapproximately 50 ms to be completed. By setting the timer in Block 867as being 750 ms, for example, the amount of time between sequentialoversensing determinations is increased so that a more appropriateamount of sensing data is reviewed before the oversensing determinationis repeated. In this way, the amount of processing resulting during theoversensing determination is reduced, increasing the battery life of thedevice.

While the determination of oversensing and the generation of thecorrected rate according to the present invention has been describedusing two sensing vectors, it is understood that the present inventioncould also be utilized to determine the presence of oversensing and togenerate the corrected rate using only one sensing vector. Similarly,while the present invention is described when utilized in a subcutaneousdevice, it is understood that the oversensing detection and subsequentrate correction of the present invention may also be utilized in knowntransvenous systems, such as the transvenous system described in U.S.Pat. No. 7,133,718 to Bakken et al., for example, incorporated herein init's entirety.

It will be apparent from the foregoing that while particular embodimentsof the invention have been illustrated and described, variousmodifications can be made without departing from the spirit and scope ofthe invention. Accordingly, it is not intended that the invention belimited, except as by the appended claims. For example, as illustratedin FIG. 7I, during the noise determination of Block 744, thedetermination is made for each channel ECG1 and ECG2 as to whether thechannel is corrupted by noise as described above. However, according toan embodiment of the present invention, once noise is determined to bepresent in either channel, No in Blocks 380, 382 or 388, Yes in Block384 of FIG. 7C, both channels are classified as being not shockable,Block 748.

If noise is not present in either channel ECG1 and ECG2, No in Block744, a determination is made as for each channel ECG1 and ECG2 as towhether the channel is in a VF shock zone. For example, according to anembodiment of the present invention, a determination is made thatchannel ECG1 is in the VF shock zone, Yes in Block 748, if, for channelECG1, both the low slope content is less than the first boundary 502 andthe spectral width is less than the second boundary 504, as describedabove. The three second segment for that channel ECG1 is then determinedto be shockable, Block 750 and the associated buffer for that channel isupdated accordingly. If either the low slope content for the channel isnot less than the first boundary 502 or the spectral width is not lessthan the second boundary, the channel ECG1 is determined not to be inthe VF shock zone, No in Block 748, the three second segment for thatchannel ECG1 is then determined to be not shockable, Block 752, and theassociated buffer is updated accordingly.

Similarly, a determination is made that channel ECG2 is in the VF shockzone, Yes in Block 754, if, for channel ECG2, both the low slope contentis less than the first boundary 502 and the spectral width is less thanthe second boundary 504, as described above. The three second segmentfor that channel ECG2 is then determined to be shockable, Block 756 andthe associated buffer for that channel is updated accordingly. If eitherthe low slope content for the channel is not less than the firstboundary 502 or the spectral width is not less than the second boundary,the channel ECG2 is determined not to be in the VF shock zone, No inBlock 754, the three second segment for that channel ECG2 is thendetermined to be not shockable, Block 758, and the associated buffer isupdated accordingly.

Once the classification of both of the channels ECG1 and ECG2 as beingeither shockable, Block 752 and 758, or not shockable, Blocks 748, 754and 760, a determination is made as to whether the device shouldtransition from the concerned state 304 to the armed state 306, Block762. The determination of whether the device should transition from theconcerned state 304 to the armed state 306 in Block 762, in addition tothe subsequent determination of whether to transition from the concernedstate 304 to the not concerned state 302 in Block 764 are similar to thedetermination of whether the device should transition from the concernedstate 304 to the armed state 306 in Block 370, and to the determinationof whether to transition from the concerned state 304 to the notconcerned state 302 in Block 372 in FIG. 7B described above, andtherefore will not be repeated for the sake of brevity.

Some of the techniques described above may be embodied as acomputer-readable medium comprising instructions for a programmableprocessor such as microprocessor 142, pacer/device timing circuit 178 orcontrol circuit 144 shown in FIG. 3. The programmable processor mayinclude one or more individual processors, which may act independentlyor in concert. A “computer-readable medium” includes but is not limitedto any type of computer memory such as floppy disks, conventional harddisks, CR-ROMS, Flash ROMS, nonvolatile ROMS, RAM and a magnetic oroptical storage medium. The medium may include instructions for causinga processor to perform any of the features described above forinitiating a session of the escape rate variation according to thepresent invention.

While a particular embodiment of the present invention has been shownand described, modifications may be made. It is therefore intended inthe appended claims to cover all such changes and modifications, whichfall within the true spirit and scope of the invention.

1. A medical device comprising: a housing containing electroniccircuitry; and a plurality of electrodes coupled to the electroniccircuitry that sense at least one cardiac signal, wherein the electroniccircuitry comprises a processor that: analyzes the at least one cardiacsignal on a beat-to-beat basis, determines a first characteristic of theat least one cardiac signal based on analyzing the at least one cardiacsignal on the beat-to-beat basis, identifies a cardiac event based onthe first characteristic of the at least one cardiac signal, in responseto identifying the cardiac event, analyzes the at least one cardiacsignal within at least one predetermined sensing window, the at leastone predetermined sensing window comprising a predetermined duration oftime unrelated to one or more beat-to-beat durations, determines asecond characteristic of the at least one cardiac signal based onanalyzing the at least one cardiac signal within the at least onepredetermined sensing window, and identifies oversensing based on thesecond characteristic, wherein the processor transitions the medicaldevice between a first state and a second state when the processoridentifies the cardiac event.
 2. The medical device of claim 1, whereinthe plurality of electrodes sense the at least one cardiac signal from anon-intravenous location.
 3. A medical device comprising: a housingcontaining electronic circuitry; and a plurality of electrodes coupledto the electronic circuitry that sense at least one cardiac signal,wherein the electronic circuitry comprises a processor that: analyzesthe at least one cardiac signal on a beat-to-beat basis, determines afirst characteristic of the at least one cardiac signal based onanalyzing the at least one cardiac signal on the beat-to-beat basis,identifies a cardiac event based on the first characteristic of the atleast one cardiac signal, in response to identifying the cardiac event,analyzes the at least one cardiac signal within at least onepredetermined sensing window, the at least one predetermined sensingwindow comprising a predetermined duration of time unrelated to one ormore beat-to-beat durations, determines a second characteristic of theat least one cardiac signal based on analyzing the at least one cardiacsignal within the at least one predetermined sensing window, andidentifies oversensing based on the second characteristic, wherein thefirst characteristic comprises at least one heart rate metric and thesecond characteristic comprises at least one of a spectral width, ametric of signal energy content, or a heart rate metric difference.
 4. Amethod comprising: sensing, by a medical device, at least one cardiacsignal; analyzing, by the medical device, the at least one cardiacsignal on a beat-to-beat basis; determining, by the medical device, afirst characteristic of the at least one cardiac signal based onanalyzing the at least one cardiac signal on the beat-to-beat basis;identifying, by the medical device, a cardiac event based on the firstcharacteristic of the at least one cardiac signal; in response toidentifying the cardiac event, analyzing, by the medical device, the atleast one cardiac signal within at least one predetermined sensingwindow, the at least one predetermined sensing window comprising apredetermined duration of time unrelated to one or more beat-to-beatdurations; determining, by the medical device, a second characteristicof the at least one cardiac signal based on analyzing the at least onecardiac signal within the at least one predetermined sensing window;identifying, by the medical device, oversensing based on the secondcharacteristic; and transitioning, by the medical device, between afirst state and a second state based on identifying the cardiac event.5. The method of claim 4, wherein sensing the at least one cardiacsignal comprises sensing the plurality of cardiac signals from anon-intravenous location.
 6. A method comprising: sensing, by a medicaldevice, at least one cardiac signal; analyzing, by the medical device,the at least one cardiac signal on a beat-to-beat basis; determining, bythe medical device, a first characteristic of the at least one cardiacsignal based on analyzing the at least one cardiac signal on thebeat-to-beat basis; identifying, by the medical device, a cardiac eventbased on the first characteristic of the at least one cardiac signal; inresponse to identifying the cardiac event, analyzing, by the medicaldevice, the at least one cardiac signal within at least onepredetermined sensing window, the at least one predetermined sensingwindow comprising a predetermined duration of time unrelated to one ormore beat-to-beat durations; determining, by the medical device, asecond characteristic of the at least one cardiac signal based onanalyzing the at least one cardiac signal within the at least onepredetermined sensing window; and identifying, by the medical device,oversensing based on the second characteristic, wherein the firstcharacteristic comprises at least one heart rate metric and the secondcharacteristic comprises at least one of a spectral width, a metric ofsignal energy content, or a heart rate metric difference.
 7. A methodcomprising: sensing, by a medical device, a first group of cardiacsignals along a first sensing vector; sensing, by the medical device, asecond group of cardiac signals along a second sensing vector;analyzing, by the medical device, the first and second groups of cardiacsignals on a beat-to-beat basis; determining, by the medical device,whether the first group of cardiac signals is reliable; determining, bythe medical device, whether the second group of cardiac signals isreliable; determining, by the medical device, a first characteristic ofeach of the one or more of the first and second groups of cardiacsignals determined to be reliable based on analyzing the cardiac signalson the beat-to-beat basis; identifying, by the medical device, a cardiacevent based on the one or more first characteristics; in response toidentifying the cardiac event, analyzing, by the medical device, atleast one of the first or second groups of cardiac signals within atleast one predetermined sensing window, the at least one predeterminedsensing window comprising a predetermined duration of time unrelated toone or more beat-to-beat durations; determining, by the medical device,a second characteristic of at least one of the first or second groups ofcardiac signals based on analyzing the cardiac signals on the schedulebased on the at least one predetermined sensing window; and identifying,by the medical device, oversensing based on the second characteristic.8. The method of claim 7, wherein, in response to determining that bothof the first and second groups of cardiac signals are reliable,determining the second characteristic comprises: determining a firstspectral width corresponding to the first group of cardiac signals and asecond spectral width corresponding to the second group of cardiacsignals; identifying at least one of the first spectral width or thesecond spectral width as a preferred spectral width; and comparing thepreferred spectral width to a spectral width threshold, and whereinidentifying oversensing based on the second characteristic comprisesidentifying oversensing based on the comparison of the preferredspectral width to a spectral width threshold.
 9. The method of claim 7,wherein, in response to determining that both of the first and secondgroups of cardiac signals are reliable, determining the secondcharacteristic comprises: determining a first metric of signal energycontent corresponding to the first group of cardiac signals and a secondmetric of signal energy content corresponding to the second group ofcardiac signals; identifying at least one of the first metric of signalenergy content or the second metric of signal energy content as apreferred metric of signal energy content; and comparing the preferredmetric of signal energy content to a metric of signal energy contentthreshold, and wherein identifying oversensing based on the secondcharacteristic comprises identifying oversensing based on the comparisonof the preferred metric of signal energy content to the metric of signalenergy content threshold.
 10. The method of claim 7, wherein, inresponse to determining that both of the first and second groups ofcardiac signals are reliable, determining the second characteristiccomprises: determining a first heart rate metric differencecorresponding to the first group of cardiac signals and a second heartrate metric difference corresponding to the second group of cardiacsignals sensed; identifying at least one of the first heart rate metricdifference or the second heart rate metric difference as a preferredheart rate metric difference; and comparing the preferred heart ratemetric difference to a heart rate metric difference threshold, andwherein identifying oversensing based on the second characteristiccomprises identifying oversensing based on the comparison of thepreferred heart rate metric difference to the heart rate metricdifference threshold.
 11. The method of claim 7, wherein sensing a firstand second group of cardiac signals comprises sensing a first and secondgroup of cardiac signals from a non-intravenous location.
 12. The methodof claim 7, wherein, in response to determining that both of the firstand second groups of cardiac signals are reliable, determining thesecond characteristic comprises: determining a first spectral widthcorresponding to the first group of cardiac signals and a secondspectral width corresponding to the second group of cardiac signals;identifying at least one of the first spectral width and the secondspectral width as a preferred spectral width; comparing the preferredspectral width to a spectral width threshold; determining a first metricof signal energy content corresponding to the first group of cardiacsignals and a second metric of signal energy content corresponding tothe second group of cardiac signals; identifying at least one of thefirst metric of signal energy content and the second metric of signalenergy content as a preferred metric of signal energy content; comparingthe preferred metric of signal energy content to a metric of signalenergy content threshold; determining a first heart rate metricdifference corresponding to the first group of cardiac signals and asecond heart rate metric difference corresponding to the second group ofcardiac signals; identifying at least one of the first heart rate metricdifference and the second heart rate metric difference as a preferredheart rate metric difference; and comparing the preferred heart ratemetric difference to a heart rate metric difference threshold, andwherein identifying oversensing based on the second characteristiccomprises identifying oversensing based on at least one of thecomparison of the preferred spectral width to the spectral widththreshold, the comparison of the preferred signal energy content to thesignal energy content threshold, or the comparison of the preferredheart rate metric difference to the heart rate metric differencethreshold.
 13. The method of claim 7, further comprising controlling, bythe medical device, a rate of determining the second characteristic. 14.The method of claim 7, wherein, in response to determining that thefirst group of cardiac signals is reliable and the second group ofcardiac signals is not reliable, determining the second characteristiccomprises: determining a spectral width corresponding to the first groupof cardiac signals; and comparing the spectral width to a spectral widththreshold, and wherein identifying oversensing based on the secondcharacteristic comprises identifying oversensing based on the comparisonof the spectral width to the spectral width threshold.
 15. The method ofclaim 7, wherein, in response to determining that the first group ofcardiac signals is reliable and the second group of cardiac signals isnot reliable, determining the second characteristic comprises:determining a metric of signal energy content corresponding to the firstgroup of cardiac signals; and comparing the metric of signal energycontent to a metric of signal energy content threshold, and whereinidentifying oversensing based on the second characteristic comprisesidentifying oversensing based on the comparison of the metric of signalenergy content to the metric of signal energy content threshold.
 16. Themethod of claim 7, wherein, in response to determining that the firstgroup of cardiac signals is reliable and the second group of cardiacsignals is not reliable, determining the second characteristiccomprises: determining a heart rate metric difference corresponding tothe first group of cardiac signals; and comparing the heart rate metricdifference to a heart rate metric difference threshold, and whereinidentifying oversensing based on the second characteristic comprisesidentifying oversensing if the heart rate metric difference exceeds theheart rate metric difference threshold.
 17. The method of claim 16,further comprising controlling, by the medical device, a rate ofdetermining the second characteristic.
 18. The method of claim 7,wherein, in response to determining that the first group of cardiacsignals is reliable and the second group of cardiac signals is notreliable, determining the second characteristic comprises: determining aspectral width corresponding to the first group of cardiac signals;comparing the spectral width to a spectral width threshold; determininga metric of signal energy content corresponding to the first group ofcardiac signals; comparing the metric of signal energy content to ametric of signal energy content threshold; determining a heart ratemetric difference corresponding to the first group of cardiac signals;and comparing the heart rate metric difference to a heart rate metricdifference threshold, and wherein identifying oversensing based on thesecond characteristic comprises identifying oversensing based on atleast one of the comparison of the spectral width to the spectral widththreshold, the comparison of the metric of signal energy content to themetric of signal energy content threshold, or the comparison of theheart rate metric difference to the heart rate metric differencethreshold.
 19. A medical device comprising: a housing containingelectronic circuitry; and a plurality of electrodes coupled to theelectronic circuitry that sense at least one cardiac signal, wherein theelectronic circuitry comprises a processor that: analyzes the at leastone cardiac signal on a beat-to-beat basis, determines a firstcharacteristic of the at least one cardiac signal based on analyzing theat least one cardiac signal on the beat-to-beat basis, identifies acardiac event based on the first characteristic of the at least onecardiac signal, in response to identifying the cardiac event, analyzesthe at least one cardiac signal within at least one predeterminedsensing window, the at least one predetermined sensing window comprisinga predetermined duration of time unrelated to one or more beat-to-beatdurations, determines a second characteristic of the at least onecardiac signal based on analyzing the at least one cardiac signal withinthe at least one predetermined sensing window, and identifiesoversensing based on the second characteristic, wherein the firstcharacteristic comprises a heart rate characteristic and, in response toidentifying oversensing based on the second characteristic, theprocessor determines a corrected first characteristic based on the atleast one cardiac signal used to determine the first characteristic, anddetermines whether the cardiac event is identified based on thecorrected first characteristic.